Ah, a budding scratch basher. I applaud you. And, as for finding those parts (you will), you may find needed things in an economical e-Bay "grab bag" offering. This will give you unneeded parts in search of a project, projects which you will find or which will find you...which will call for more parts. Ask me how I know.
BTW - on the PRR "PW K4" project (never did it; I moved out of PW long ago), don't forget that the engine comes with a very un-PRR pilot (in fact, it's NYC-ish, if anything) that will need a little fiddling.
And off you go. Probably the best fun to be had in this hobby - at least it is to me. I'm always assaulting something; I've heard that some guys actually operate their layouts. Weird.