Here's th eperson I dealt with to buy extra sensors- they were totally reliable with my purchase:
eBay Motors: Backup REVERSE PARKING 8 SENSOR LCD/SOUND 8 SENSORS (item 360068542528 end time Jul-15-08 02:03:37 PDT)
You will either need to buy 4 additional LONG wire sensors, or extend the kit sensor wires yourself, as 4 of the sensors come with short wires. The long ones are 20ft., the short are more like 8ft, and won't reach anything on our trucks.
-Dave