I searched, and found a lot of posts about needing references, but if you don't have any and you see a provider, is it okay to ask for one? If so, what's the proper etiquette to go about doing so?

or by email afterwards.  I never had a gal refuse to do so as they know references are a vital means of keeping themselves safe and sound.

What I would not do, however, is ask a gal for a reference if too much time has past, say 3 months or so.  At that time distance, the gal may not even remember you that well.

a verification site like DateCheck, Preferred411, or Roomservice2000.  DC and P411 are pretty much nationwide while RS2K is more popular in the midwest, particularly around Chicago.  P411 also lets their providers indicate if they are newbie friendly, and those are the ones you should look for, screening is somewhat lighter with them.  As previously mentioned, your age may limit the ones who will see you, you'll just have to keep looking until you find one that does not have lower age restriction.

... a new friend, I always tell him (usually when we are both freshening up after our romp, or in my thank you note), that "while I have enjoyed your company, and would love to see you again, should you wish to visit with another lady, please know that you may list me as a reference".

I'm registered on RS2K and P411... but the local girls don't use either.

I hope the ladies I'm planning on seeing in the near future are like that.

...feel that the lady will have hurt feelings should he ask her to be his reference.