JL Audio, MB Quart, Kicker, Kenwood, pioneer, sony, Rockford fosgate, alpine all make subs. if you need to go cheap go rockford fosgate. make sure the box is built to the speaker, not something premade.
It is possible to make a cheaper system sound good, you just have to work a little harder. amp wise you have Rockford fosgate, JL audio, sony, alpine, kenwood, pioneer. Most have different series so you should be able to stay in budget. Better to be overpowered than under powered on the amps.