Try posting on the local board for your area asking this.  You can also run a search and look for ladies that do couples/threesomes,  then contact them and see if they into what you are suggesting.

...and had a great time.

I began by searching local advert boards for providers who specifically mentioned "couples".

The only thing that's hard about it is finding one that a) you like, b) she likes, c) who entertains couples.

I think it took me about a month to find one. She was actually "visiting" rather than a local.

Re:Threesome with provider and my SO???

We've done it several times, in several cities.  It's easier in a city with many providers (LA, San Francisco, and Las Vegas for us, all more than once with different providers!).  Just check the reviews for "Two Girl Action" (you're looking for "Yes - really bi").  Also check their web sites for "Couples" rates (sometimes more than the single rate, but not always).

I suggest you let your partner take a look at the photos, read the web site, and even read the reviews, and take the time to choose someone who appeals to both of you.  Besides, as with finding a provider for myself, finding one for both of us includes the fun of the hunt!

Good luck.

Paul