Where can I get a cheap low-power amplifier?

http://www.markvelectronics.com/store
There are very few available. One source is to buy a cheap boom
box and only use the amplifier. Another source is Radio Shack.
A third alternative is to buy a car stereo booster and get a
12V power supply for it. Finally, you can build an amp pretty
easily if you are handy, but it probably won't be that cheap.
Mark V Electronics, for example, sells 20 watt amp kits for
under $30 and 80 watt amp kits for under $150. Sound Values
has a 60 watt amp kit complete for about $200, and Old Colony
sells some amp kits for a bit more. All three, Mark V, Old
Colony, and Sound Values kits have been built by satisfied
rec.audio.* posters, although quality of the Mark V kit is
lower than the others. (See 11.15, 11.16, 11.17)

Mark V Electronics, Inc.
8019 E. Slauson Avenue
Montebello, CA 90640
http://www.markvelectronics.com
Tel: 800-423-FIVE (orders outside Canada)
800-521-MARK (orders in Canada)
213-888-8988 (catalog/info) Fax: 213-888-6868