Combatman, that is the one problem that has many possibilities. Sometimes, you can contact the original designer of the plane and they might have the parts made or has the molds to make some for you. As far as in cowls, Fiberglass Specialties is one company that makes cowls for various kits and ARFs. For some planes, you have to look for a similar plane to get the parts you need (such as a canopy from an ARF or wheel pants). Finally, you will need to make them yourself using various techniques. Sometimes when you build a plane from scratch, you are making all the parts from scratch.

Happy Flying and soft landings!