Another option. I bought a pair of Polk Audio - 10" Dual-Voice-Coil 8-Ohm Subwoofer - Black from Best Buy when they were on sale. I' d be happy to sell them for what I paid- $33/ea plus shipping. Let me know if interested.
I did not use them since I could wire only 1 or 8 ohms. If you find a 1 ohm stable amp like in previous post, all you would need to get is a box.