Research and reviews are your friend! You can use the advanced search function to filter out reviews by "yes-really bi." Also, tryst and eros have filtering options for providers that are available to women. That is a good place to start. Next I would go through a provider's website, social media and OnlyFans. Some providers are pretty open and explicit about their sexuality and will openly broadcast their preferences. Some also lie, so sifting through this stuff helps. Does their website have information about couples/women? Do they list duo partners on their website? Social media can also help gather information, posts around Pride month can be informative. Some providers also do date women and will talk about it. If they have an onlyfans, that can help as well. Do they film G/G content and seem to enjoy it? I'd also recommend posting in the regional Houston/Dallas/Las Vegas boards and say you are looking for a bisexual/lesbian provider for your wife and see what you get.
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Word of caution on the Las Vegas market. There's a lot of smoke and mirrors. Plus the market is tourist heavy so providers aren't as reliant on repeat customers. So... Buyer beware. I'd recommend going to one of the brothels. Your wife can talk to the girls and flirt with them to get a feel for it before hand. In person is always the best because you can ask questions like "do you enjoy women" and follow up questions, like "what is your experience" and "what do you like to do with women" to see if they are faking it or not.

and her profile says she's "Really Bi."  Her site also says she has "hot girlfriends." So......

Do you have p411? Not fool proof but it certainly helps a lot. Most of the ladies have their preferences listed.

Thank you for the thoughtful replies. We are definitely doing our research. We want this first time to be perfect!

I do not have p411. I used to and remember it had that info.
Tryst and sinsations do not. Or at least we don't see it.

Good suggestions so far.  A few more but I'll begin with the warning that affects almost every "check box," specifically: YMMV. A hostess might be truly-bi and have great duo reviews and so on but there's always the chance she might say, "I guess I'm not really into you." (Too tall, too short; hair too long, too short; too rough, too sub; You're not my type, I guess I'm not your type; etc.)
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(Turnabout happens to guys, too. "But your Profile says BBBJ-CIM-Swallow!" "Sorry, I don't like your [body odor, eyes, hair, girth, choice of clothing, ...]."  
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I vaguely recall some years-old posts that suggested meeting for a drink or something to let the ladies figure out if they think it will work as hoped.  It might be as simple as a conditional: (a) Meet in hotel bar for a drink for (prorated or discounted regular rate). If the wife likes the vibe [it is HER decision!] (b) go straight to the room for a session OR (c) agree to schedule a BCD meeting in the near future.  
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What is your home city or where do you want this to happen? Search the local discussion board to look for (local) advice. Search the local buzz board to see who is currently available: see what they say about themselves, bi-wise.  
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Under "Reviews", use Search or Search New or Recent Reviews for your city, other prefs (height, shaved or not, etc.) and select "Two Girl Action [Yes - Really Bi]." You might get some false positives and you will miss those are Really Bi but don't have that indicated on their Profile.
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When searching reviews for doubles (2 Providers who are known to work well together + client), search a name and the & symbol. "Mary &" will return:
Mary & Nancy
Maryanne & Ginger
etc..  
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You can also search "Mary and" but you will get:
Mary and Nancy
Maryanne and Ginger
Maryandrea  
Mary Sandra Smith  
etc.
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(When submitting doubles reviews, USE "&"!!! Do not use "AND"!!!)
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Good luck.

Per the googles 1 in 18 women in the US identify as bisexual. So statistically if you contact 20 sex workers you should at least find one that is "really" bisexual. Now if you contact 20 sex workers how many will say they are "really" bisexual because they need the business vs. those that will be honest and say no? That is a very good question.....
Also curious about your above story in Vegas, did you or your wife ask the dancer if she was bisexual? The reason I ask is because when I worked at a legal brothel in Reno we did have several couples that came in and would chat with all of the ladies working. One couple was actually quite taken aback when I told them that I was not bisexual, they assumed we all were. We did have a female that would come in as a regular and she would always ask the "ladies that were not into ladies" to not approach her so as to not waste her time. This was fine as it honestly eliminated about 3/4 of us that were working and we did not want to have to tell her we were not gay. I would say as a whole sex workers probably have a higher propensity to be bisexual or at least heteroflexible vs. the general population. Will you and your wife maybe get someone who is just "gay for pay"? I guess that will depend on how good of an actress she is!! Be honest, ask questions, and best of luck!!

.... I have been very fortunate over the years to never really been disappointed with a provider. Like others have mentioned, I'll look for positive mentions of entertaining women preferably on their personal website. Many providers are genuinely into women, but it can take some homework to narrow down the list. I'm also a big visitor of strip clubs, and with experience came the ability to find dancers that are more than happy to entertain a woman customer. That being said, I've gotten some amazing VIP sessions with dancers who proclaimed to be straight so I never rule anyone out these days. My situation is a bit different as well since it's only me pursuing a woman, rather than a couple which can make it a bit more difficult. Either way I hope you find your perfect lady friend!

I frequently play with a female. She is bi in the bedroom and has stated she wouldn’t be in a relationship with a woman. We know a few bi providers. Most have told us the same thing. However, we do know one who prefers to play with women in her private life. This site and p411 are invaluable tools. Then when you develop a repertoire with some of the ladies. They usually point to other friends with similar interests.

Queen Bia can help you out with that. If I was looking for such a rendezvous, I wouldn't look elsewhere.