Depending on the impedance of each of the speakers, one option could be a single 2ch amp. If they're 8 ohm or higher, you could wire a pair together (parallel) to each channel. If they're 4 ohm, you could wire them in series.
If you need 4 distinct output channels for a quadraphonic surround, then you'd have to have a 4ch amp.
2ch options.http://www.parts-express.com/pe/showdet ... er=300-380http://www.parts-express.com/pe/showdet ... er=310-300
Two of the 2ch options ;-)
Anyway, here's the bad news. Any of the options listed above will not control the volume of the sub. This is because the amp for the speakers works independently of the amp for the sub. The only common volume control for them is the computer, so if you turn down the amp for the speakers, the sub will continue to pound away. The only way to make them work together is if you have a common volume or gain control. This can be accomplished with an A/V receiver but in order to get something around $50, you're looking at the used market.
So you either have to suck it up and adjust the volume with the computer, up your budget so you can buy a new AVR, or keep an eye on craigslist for a while till something pops up in your area. Good luck.
Side note, there is this option but I don't know if the sub amp will be strong enough for you, because I know it won't put out the rated power.http://www.parts-express.com/pe/showdet ... er=310-308