I got them from Hortons Hot Rods. Several colours to choose from too. Here's a copy paste from the Horton catalogue.
SEAT BELTS - by Allstar 2-point nylon webbed lap belts.
manually tightened.GM style buckle. Sold WITHOUT mounting
hardware. Colors available: Black, gray, tan & Charcoal Sold
each (x) denotes colour
#ALL9811x $25ea
I've been running Juliano's for about 10 years and now have them installed in Dog Spit. Their lap belt sets come in a variety of colours and sell for $19.95 per seat. If You're not bolting into the frame, I urge you to use their mounting plates. they are truly the safest way to fasten seat belts to the floor. They also carry a vast array of retractibles and shoulder.
Here is a link: www.julianos.com/catalog.html