Try onlinecarstereo.com or sonicelectronix.com.
I got a double-DIN JVC head unit with dual USB, Bluetooth, and AUX for $135 refurbished from onlinecarstereo.com and it came with the mounting kit and wiring harness. I had to purchase a wiring harness separately because of my factory sub. But it might be worth checking out.
If you have a factory sub, it will be in the trunk/hatch on the left side.
older: 2002 ZX5//old: 2006 ST//current: 2015 FiST