First, be aware that you can use almost
any penstyle atomizer and cartridge with the BE112 battery or pass
through - DSE801, RN4072, Joye302. [The exception is M201.] My favorite thus far is the Joye302 (but don't tell BE112-loving Sun Vaporer; he'll just call me names

).
But to answer your question directly (and keep Sun off my back

) I know of two U.S. suppliers of BE112 kits and parts:
Electronic Cigarettes of Houston and
The Vapor Spot. I assume you already know about
BestEcig, the retail website (in English) for the Chinese manufacturer itself.