hifonics hifi subs are popular and cheap for this.
What amp do you have?
Phoenix Gold V1502 - Phoenix Gold Ryval 1000 Watt 2-Channel Amplifier
■Ryval Series 2-Channel Class AB Car Amplifier
■Max power output: 1000 watts x 1 chan.
■LED power and protection indicators
■Remote Monitoring Display (RMD) Port
■2/1 channel operation
■Speaker-level input
■Preamp RCA outputs
■Extruded aluminum heat sink
■3 Position crossover configuration switch
■Switchable high-pass filter (100 Hz, 12 dB/octave)
■Variable low-pass filter (40-400 Hz, 12 dB/octave)
■Frequency response: 20-20000 Hz
■Efficiency: 100 dB
■Dimensions: 13.15" L x 9.30" W x 2.25" H
■8-gauge power and ground leads recommended
■Amplifier Class AB
■Number of Channels 2
■RMS Power (4 ohms) 150 watts x 2 channels
■RMS Power (2 ohms) 225 watts x 2 channels
■RMS Power (1 ohm) N/A
■Bridged RMS Power 500 watts x 1 channel