An ATX power supply should give you what you want. You can find several ATX to banana post breakouts online:
http://www.seeedstudio.com/depot/ATX...ly-p-1222.html
If 15 amps is your power budget, you're pretty much set with any $30 supply you can find online.
If your budget is higher than that, you'll need to check the specs for the supplies, which should be easy to find as the beefier video cards require certain amounts of power delivery.
If you need even more than a single PSU rail can deliver, you can find multi-rail power supplies. In that case you need to remember you can't connect the 2 rails in parallel, they must be hooked up separately. Switch mode power supplies without explicit support for synchronous power conversion don't play well with each other.