You might be surprised actually. There are plenty of decent saxes just sitting around, tarnishing. You might get lucky at a yard/garage/church sale, or even a local dump might have a swap shop. You could try craigslist too. Another place you might look is the music department at a local university. People there are bound to know where to get decent stuff for a decent price. For a first sax, a couple of dents on the body are ok, as long as the metal is intact and the tube of the bell isn't deformed.

If you get a used one, go and get the pads replaced and have it cleaned up and have the guy make sure that the cork on the neck is ok and that all the little bars and springs are doing what they're supposed to. It looks really complicated, but it's pretty easy to know when something isn't working on a sax.

Basically, the keys that the pointer, middle and ring fingers can naturally reach on the left and right hand should close valves, while the keys pressed by left thumb, the palms and the pinkies, should open valves and no valves should be flapping around.