Besides used Winchesters and Brownings mentioned above, there are used SKB (IIRC, models were 100 and 200??). You might get lucky and find z used CSMC RBL in 20 for about 3, very possibly a used AyA 4/53 or a used Ugartechea. The issue with the last two will be weight as they are designed for field use (like most high quality SxS guns).
After that will be the newer Turkish imports, mostly from Huglu and AKUS which will be the S&W Elite Gold , now sold by Cabelas as the Dickinson, and the CZ guns.
After exhausting those will be older Stevens/Savage-Fox , older US guns like Parker, LeFever, et al. With the last few mentioned ones, I see two areas of potential concern - their age and condition in your price range and the stock dimensions back then tended to have a LOT more drop. They were also most likely used for hunting only and could have suffered from serious use. Parker, LeFever, the original AH Fox, etc., all have web sites dedicated to those guns with lots of knowledgeable folks. I would also look at :
http://doublegunshop.com/forums/ubbthreads.php for any info about SxS guns.
Good luck in your quest!
PS - JMO, but if your interest will be mostly clays, in your budget you will find a lot of choices in O/U's from the likes of Beretta, Browning, SKB, possibly a used Geurini.