Well, yes and no. You can check Cityvibe, Eros, your regional TER board or even CL or Back Page and start making phone calls.  Keep in mind however that "immediate" availability is going to mean she doesn't screen and that is always risky.
I would be more inclined to try an agency with well reviewed ladies if I wanted immediate nooky.

Thats kinda funny.I guess some guys think we sit around in thigh highs and heels all day.lol.It's always best to give notice or plan ahead.Don't you want her to look her best 4 you?Although you might get lucky.All you have to do is call and ask.good luck!Have fun

About the only "immediately available" gals are at AMP.  If that is what you want, go for it.  Otherwise, plan ahead a few hours at least.

I wouldn't want to see a provider I haven't seen before who doesn't screen and would just show up on your doorstep at a moments notice.  It tells me that she either doesn't care enough about her safety, is perhaps a reckless about how she conducts business, or could (although unlikely) be LE.

The only people I'll call for availability on short notice are providers I've seen several times who have already screened me.  Even then I'll always ask for to see them within the next 2 hours or later in the day/night.  You don't want a girl showing up in rushed mode.  I like them relaxed, refreshed, and ready for some sexual healing