Marketplace topic.
EDIT:
Okay, whole Proton Packs rarely come in a ‘easy to build’ set, a lot of the time there's only a few specifically mass produced parts, you might find some clippard valves, or a cast of a bumper (For details on the names of the parts, visit either
Ghostbusters Props or
The Ghostbusters Project) But more cases then not you will probably have to build your own Proton Pack, as parts manufactured by fans are hard to come by these days.
Failing that, there is eBay.