many providers websites specifically say they take wardrobe requests.  But there is always the possibility that they don't have that item.  For instance, I once had a guy who wanted white closed toe fmp's.  I don't own a pair.  I have them in black.. lol

If you want an outfit that she wore in one of her website photos, feel free to ask, she will likely oblige.  They will usually try to accomodate reasonable requests as long as you aren't asking them to parade through a hotel lobby looking like a ... well... hooker...
If you are looking for something more naughty than would be appropriate in public she might well be willing to change into it after she arrives.

If it's something very specific, or really unusual, she probably won't have it and you'll need to provide it.  Advance notice is always best for something like this.

If it's something as simple as "black fuck-me pumps with an ankle strap, and black fishnet stockings", she probably won't care.  If it's something like "A WWII German Army officer's coat, worn over a Metallica T-shirt with dried bloodstains on the front, thigh-high stockings with 'Hell's Belle' written on them in Gothic script, purple-dyed ballet slippers, and an 18-inch coal-black strap-on dildo", then she might tell you to find someone else.  

not on the 2nd call of a two-call system.

As all of the above indicate, ask ahead of time, keep it fairly simple... one thing that many ladies do like is for you to make a request like this and bring the item(s) along with you as a gift...