it really depends on what you want. do you want something that looks like original shelby style belts? or some aftermarket?
the shelby style ones are going to take some work and will need to be custom made. use a 70 style lap belt retractor with the slotted hole for the attaching the seperate shoulder belts to and have an upholstery shop custome build the harness with seat belt webbing and a leather "y" attachment or spend more than 1500 bucks to get a set of repro shelby belts.
or you can look for an aftermarket 4 point style harness with a tensioner/retractor. i've seen these somewhere before just can't remember where. maybe Corbeau?