I didn't buy a kit. I bought a power door lock actuator from Ebay for $6 (it's cheaper now) and a few spools of wire, some split loom, and various connectors. The most expensive individual part was a piece of aluminum stock that ran me about $8.
I followed the instructions on this thread:
http://www.f150forum.com/f38/keyless...-73513/index2/
and it came out perfect. I think some poeple have put together kits and are making a quick buck off of that.
Good luck if you do the modification. I find it really useful. Feel free to PM me if you'd like more info.