etrailer.com, $16...I've yet to be in a autodroid store that doesn't have something like this on the shelf, mine has the 4-prong built in to the adapter, I have another where it's just wires, which may work better for some.
if you are replacing the separate turn - stop bulbs per the stock lights with one single combination light, then a harness adapter is required, trailer style. I actually connect mine at the firewall and ran new trailer wire to the back, used the "ground" wire in the harness as the backup wire.