Any parts store or Radio Shack should have them. For a four headlight system like yours, with no fog lamps, you'll want two relays, one for the low beams, one for the high beams.
Run a wire (with a fuse) from battery power to the #30 terminal on each relay. Connect the low beam wire (purple?) coming from the dash to the #85 terminal. Connect #86 to ground. Connect #87 to the purple wire that feeds your lights.
The brights wire the same way, just with the red (I think) going into the #85 and out from the #87.
Check your headlight sockets. The outside ones will have 3 wires, the inside will have 2. Whichever color is missing from the inside is the low beam.
Check out Goody's website for an overview on this...
http://www.goodysgotacuda.com/Headlights.html