if you just want to test speakers than get a cheap plate amp from http://www.partsexpress.com). if you want to test amps than get a car battery charger(the higher amperage the better) and hook it up to a regular car battery. I used that setup to run near 1000watts to 2 15"s in my room for a while. Dont have pics on this pc though. The computer power supply idea is a bit more complicated and you might need to link a few supplies together, but a simple google search brings up everything you would ever need to know. Heres a video from youtube, theres dozens.
If money isnt an issue than you can buy a few of these and link them(depending on how big of an amps you will be testing) http://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?ProductID=19564 or if your good with electrics there are schematics for 12v 100amp(and upwards) schematics....It all depends on how much power you plan on running and for how long they will be playing! Hope this helped.
If you do decide to run a psu and the output isn't enough to power your amp this is a great tutorial on how to run multiple psu's strapped for more amperage.