I have got a few bargains off "poor mans watch form" sales corner.
As with buying anything on the internet buyer beware but I have not had any problems buying from the private parties there.
PMWF
I also have bought from these guys, they have a lot of vintage stuff:
http://www.whattimeisit.net/
Most of the Omega seamasters I have seen (I would like to have one too) are above your (and my) budget. YMMV