the most popular epoxy topcoats are Devcon 2-Ton epoxy (also used not only as topcoat but to glue in hardware and such), Envirotex Lite, and Flex Coat.
you can get Devcon 2-Ton at a hardware store, you should be able to find Envirotex Lite at hobby/craft stores, and i'm pretty sure you have to buy Flex Coat online.