Just about any auto parts store carries Dupli Color. I usually wet sand with 600 and then use their Clear over the final job. It's not needed, but if you do multi colors, the wet sanding will knock down the crisp edges between them and the clear will also help protect the finish. Just use light tack coats initially and slowly build 2 or 3 coats.
If you have an air brush, Auto Air Colors has some really fantastic water based automotive grade paint that pros are using for custom paint and graphics. It, too can be cleared with Dupli color clear on a model. It can be purchased in 4oz. bottles, too. It's not expensive...
http://www.autoaircolors.com/index.htm
