This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
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.
Sound Values has a 60 watt amp kit complete for about $200, and
Old Colony sells some amp kits for a bit more. These kits have
been built by satisfied rec.audio.* posters. (See 11.15,