You can use a single stage from PPG and get a clear with compatable activators and you can lay your single stage and put the clear over that and it's like base coat clear coat but cheaper and still not bad lasting. Some people have their oppinions on this option but we do this at the shop on some old cars when the guys aren't too worried about 8-12 years down the road what the vehicle looks like.
Or you can check out www.southerpolyurethanes.com and they have a few color choices and primers, clears, etc for a cheap price but the product is GREAT. That is a base coat clear coat deal there but I think they have single stage as well. It's kind of a new company (not that new anymore) but that is one reason their price is low I am thinking and customer service is very good.