I can't speak for how it is elsewhere, but in my neck of the woods there's a definite stratification in the levels of musicianship and the methods of hook-up.
In general, the ones good enough to be session musicians all pretty much know each other, if not personally, at least by name or reputation. There's a definite, if somewhat loose, network between the more experienced musicians.
These are the guys and gals that one needs to tap into as a source for session musicians. The greatthing is once you can tap into it, they can be relatively easy to hire because if one guy can't do it, he can probably name 2 or three other great guys as alternatives that you can call.
These people are the type you have to go to, however, they will not come to you. They can get work without you. They are not the type to answer ads in the local entertainment paper or to pull phone numbers off of ad flyers hanging in the lobby of the local Guitar Center. They are not looking for a band or for musicians for their band; if they need that they can usually find out whats up on that scene in just a couple of networked phone calls. If they were a family there'd be so much incest and deformed babies that even a trailer park would kick them out
Unless one gets real lucky, kind of player one attracts from a "session musician wanted" ad hanging on a bulletin board or through an Internet collabration network is not typically of the caliber one would really like to have on their record. I'd say it's about a 1-in-10 shot.
That's how it is around my orbit, anyway. I can't speak for elsewhere.
G.