If you are hardcore, you can run just straight flat black primer. Doesn't provide much protection at all though.
They also make flattening agents that you can add to the clear so you can make any color flat or matte. Best option by far.
Second place would be a single-stage flat paint which requires no clear. Hot Rod Flatz makes a wide range of single-stage flat paints that look great and are dirt cheap. Definitely the best bang for the buck.