Finding Studio musicians online

I am aware of Studio Pros, and their service.

I was wondering if there are other options if I needed a real sax or guitar player on a track. Is there a website setup to connect people like this. Essentially I would send, lets say a guitar player the complete song, minus the guitar parts, he would add it, and then send me his guitar track.

Anything like this would be very helpful.

Thanks in advance!

Chaz