just lap belts? if so then check out summit racing, BUT its about the same cost to buy a complete 5 point harness, and you can use only the lap belt part, then if you want to get serious you have the rest of the harness too...
Is passanger class ok? I got some replacement ones from Mepco a while ago. For the back seat in my CJ8. I bet you could get some at any autoparts store.
Or better yet, see what you can get at Pick and Pull?