Be careful - making replica props can become addictive!
I don't make the full packs for anyone but myself - I am still just learning about all of the incredible building and construction process, but have built a lot of electronics that go into other enthusiasts' packs.
A couple of great sites for related items:
www.mattycolletor.com for Mattel made items for adults (I think), They have offered PKE Meters, a Ghost Trap and also Ecto Goggles with video animations you see through the lens - a Head-Up-Display?!
www.gbfans.com for a community of Ghostbuster enthusiasts that offers lots of info, technical support and a wide array of items that can be purchased or comissioned to build.