Found this Ghostbusters costume with Proton Pack and Gun for sale on eBay in the UK that might be of interest to someone. It seems to be home-made with the backpack being made out of a cardboard box but the suit itself looks pretty good and overall it's not too bad an effort!

http://cgi.ebay.co.uk/Ghostbusters-Costume-with-Proton-Pack-and-Gun_W0QQitemZ8372932551QQcategoryZ52766QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Check it out!