most of those audio stores simply use power supplies that are the same as the internals of a battery charger. Becasue a battery charger is just a transformer/rectifier, that turns the 120 volt AC house voltage to 12-14 volt DC.
I have used one before to test my amp and other 12 volt equipment like starters, lights etc. so it will work.
BUT you can not use a automatic charger, you need one of the old fashioned kind, and make sure it has enough capacity to keep up with the power demands of the amp. An automatic unit somehow can tell wether it is hooked to a battery or not, if it is not hooked to a battery it will not allow current flow.
You could go to a place like Radio Shack and get a power supply but I haven't ever seen any there for over 20 amps, but maybe if you ask they could get a hold of a higher capacity unit.
