What RFID options exist for iOS devices?
What type of RFID tags are you trying to read?  For a motor pool, I would expect you are looking for UHF Gen 2 RFID or even active tags (like Bluetooth LE) as opposed to NFC where you would need to be standing right at the tag on the auto to read it.

We are developing a UHF Gen 2 RFID system for iOS/Android.  The device, unfortunately, needs to be larger than a square since the 10' read range of UHF Gen 2 RFID requires a bigger antenna. Our dev site explains this.