I know that you're trying to stay away from a computer power supply, but they are pretty much ready-made for such a simple project, and dead-simple to strip down to the essentials. They are rated for a significant load (all things considered), and, after removing the unnecessary leads can be made fairly compact. They can be had free or close to all day long on Craigslist and e-scrap locations.
As mentioned above, a "wall wart" 12v adapter is also a good option, though the amperage rating of a stereo, even without an amp is in the 10-20 amp range, and most wall adapters are only a few amps at best. A trip to the nearest flea market, yardsale, or thrift store will likely turn up a few options.
Third, if you have components readily available and are handy with a soldering iron, there are numerous guides (instructibles comes to mind) to scratch-build your own 120-12v power supply. The rectifier alone might end up costing as much as a used PC power supply though.
Good luck!