Radio Shack used to have some audio power displays; there's a couple on eBay now. The Velleman kit K4307 looks just about ideal: it displays up to 2000 watts/4 ohms, and doesn't need an external power supply. It's mono, and sells for about $40. Just google K4307 and you'll find a bunch of places to buy it. The manual has the schematic, if you wanted to build one from scratch.
For line level display, there's the "American Audio" dB-Display, available new from $60.