[Toa Circuit]

Older computer PSUs can output 20 amps of 12 volt power for cheap if you know how to hack them. Check your local junkyard.

You could possibly take an old UPS and use it. Just draw off of the battery terminals. The battery will act as a high capacity capacitor of sorts, so you can achieve high amperage output, but not have to worry about swapping in and out batteries.

But of course, all of this depends on your exact needs- how many amps you need, if that is continuous or not, and how much of ripple is acceptable.