Quick question about airsoft?

Alright, I have brand new internals in an echo 1 M4 Commando. However, the externals one could call, well, less than stellar. The Collapsible stock only goes to two settings, the front hand-guard is broken due to wear and tear, the dust cover does not work, etc. My question is, is it possible to get a shell of an airsoft M4 commando,without having to buy new internals? If so, where, and which shells are compatible?