Here is the one I got. To use bluetooth OBD2 readers on the iPhone you need to jailbreak it. WiFi ones work but are much more expensive
Amazon.com: ELM327 Interface Bluetooth OBD2/EOBD Auto Scanner V1.4: Electronics
Only thing I don't like is that it is a little large and gets in the way a little in between your legs when installed (have to be careful not to hit it when getting in and out), so I ordered another low profile one. Should be here soon, I'll let you all know how it works!