Here's a description of enamels and lacquers I found on another hobby board (not my words, but good info).
Enamels go through a "cure" process where the polymers in the paint set up longer chains and bond together. Enamel, once hardened, won't soften back up. So laying another coat of enamel on, you want to scuff the existing coat for good adhesion.
Lacquer dries by the evaporation of the solvent. Lacquer will soften the existing coats slightly, and you get a bond between coats that way.
As far as where to find Dullcoat, anywhere that sells Testors Model Master paints should carry it in the little spray cans.