well... you can get lacquer-based clears in spray cans from pep boys or the like. however, if you're thinking about doing a car, particularly in entirety, you'd probably want something a little better. it can be rather expensive. the only place i can think of off the top of my head would be house of kolor -- they make all sorts of paint products from primers to kameleon colors, but it's a bit steep. still, might be a good place to start looking/learning, or to find good search terms for google or whatever:
http://www.houseofkolor.com/hok/index.jsp