I would go to
www.madisound.com and check out the Vifa 10" sub on the "on sale" page. It is just under 50.00 and would make a very good woofer that wouldmatch up well with the components you have. Another option that could be even better would be the Martin Sound 2601 from
www.martinsound.com. It is also a sub-50.00 sub that would be hard to beat for under 50.00.
Both would work well in a 1-1.5 cubic foot cabinet. Good luck.
As far as an amp goes, I would go to ebay and look around for a 50 watt per channel amp by a company like PPI, RF, Phoenix Gold, Alpine etc...that is if you don't mind buying a used amp.