I hope it won't be used to shoot up a centre for ants!
bobsanto: honestly, the fact that it's clear plastic with non-functional furniture, is what's going to make this hard to find parts to build this. I don't think there's a company that will mould this for you, for less than the cost of a proper metal one. Not to mention you'll need to source the internals to make it shoot.
The closest you'll be able to get, is to get a non-clear plastic M4, or a tinted lower receiver M4s, with functional stock and rail. Then attach an M203 to it. The M203 may or may not be functional, but it will likely be an M203 that was functional at some point in the past.
One solution would be to go to Walmart, and buy one of those clearsoft M4s for the base gun. Then tape a few toilet paper rolls together, wrap some pipe cleaners around it to create the bumps, and then spray paint it all black.
But to answer your question: you probably won't find any place to build you a clear plastic gun.