What kind of chain to buy for internal gears?
The chain you need is 1/8 inch chain, usually sold as singlespeed chain. 

A derailleur chain, which are sold as multi-speed chains, is narrower and the cogs may be too thick to fit.

EDIT: As said in the other answers, some singlespeed and internal gear hubs have narrower cogs and can use the 3/32 inch chain. An 1/8 inch chain may be slightly noisier with these cogs, but works.