There is one reloadable engine case that I have seen used w/ black powder, the company will not endorse this and claims the casing can explode.
http://www.apogeerockets.com/Rouse-Tech_Motors.asp
EboNE- most hobby shops carry the rocket kits, some do not carry the engines. The real good stuff along with the engines can be found online.
The link Tom posted is awesome for the kits but do not sell the engines.