Ryan,
Yahama, Behringer, & B-52 all make some real killer sub boxes that are affordable.. contact me by e-mail directly and i'll let you in on where you can get a killer deal on them... or if you have an amp with a built in crossover I have other solutions...
Good Luck,
Brian Douglas
Sprocket Audio Southwest (Pro Audio Engineering & Karaoke Consultants)
SprocketAudioSW@aol.com