Beware of pawn shops! As Nelskie said, most of what you're gonna find are gonna be pretty worn and in need of maintenance... The internet has made finding the "great pawnshop deal" more rare. Most of the time, the pawn shop will mark the price of a used item at the same price level of a new one, trying to get the public to pay more. It's gonna take a lot of looking. If you find a good amp, do your homework and check to see what they're going for used at a music store, or check the 'Bay. Also, go to some music stores and check out new versions of these amps so you know what they are supposed to sound like, especially with tube amps, as those suckers require a little more upkeep (replacing old or worn out tubes). It might be a good idea to find out what the cost would be to replace the particular type of tube the amp uses...
Don't be afraid to haggle! Most pawn shops expect this, which is another reason why the prices are high. Common responses to initial offers are: 1) a ton of 'tude, like you're insulting them, and 2) "I dunno. I gotta check with my manager." If you get the attitude, walk away...
It's also a good idea, if you buy anything from a pawn shop, to run by the local police agency and have 'em run the serial number to see if it was stolen. If the amp is suddenly available, looks brand new, and is 1/4 the price of the new model, it might have been dumped from a burglary. Nothing worse than buying a used Fender Twin at a great price and finding out it's no longer yours...
I've found some good deals in the past in pawn shops, but it took a lot of work...
Guitars:
Fender 2006 MIM Fender Stratocaster HSS in 3TS
Ibanez RG 570 with a bridge Invader
ESP M II Deluxe with a Tune-o-Matic bridge
Eleanor, the magical, mystical Road Worn wonder Tele
Blackstar HT Club 40