Any ideas on making seat-belt buckles?

Don't know for sure about the seat belt buckles except maybe try to form them out of aluminum foil. Something that small might be hard to get looking right. I would think that for the $5 or so that a pe set costs it would be worth it in this case. Plus you get a whole bunch of them. Heres a page that has some nice tips on getting the belts and other stuff looking good though:
http://www.briansmodelcars.com/tutorials/tutorial.asp?TutorialID=27&CurPage=1

Where are they 5 bucks at? My local store has them for $13

They have them here but are out of stock right now. http://www.scaledreams.com/DETAIL.HTM look under pe acc.

You might try a google search for the Detail Master set though.

Model Car Garage has a set for $6 here http://www.modelcargarage.com/mcg_display_item.cfm?m=2089

Hope those help you out some.