Ghostbusters.org

Okay, I know alot of people use trash cans, but I've found a better method that looks great! Get some 8-10" diameter duct pipe. Cut it to the wanted length. Then get a rubber bouncy ball (Easiest place to find them is any Toy Store) The ball needs to be 8, 9, or 10". The ball and the Duct pipe need to be the same diameter!!! Get 2 of the balls that are the right size. Next, get some rubber cement, or hot glue, and put a ball in each end of the pipe, each ball halfway covered by the pipe. After your adhesive dries, cover the seams with putty, sand it where there isn't a seam. Next spray paint it all grey in a ventilated area.
For the green side tanks, you can either use 4 coke bottles (Cheap way works great ;) ) Or, if you want it stronger, look for some of those covers that go over poles in front of stores. I don't know what there called, but you know how most stores have 4 posts in front of stores to keep cars from driving onto the sidewalk. I remember a website that had different sizes of these covers. Get 4. (So you can cut off the 2 extras round ends and make the other 2 round at both sides) I forget the name and website, you'll have to do research for that. The gun, everyone knows the crayon bank thing, but if you can't find one, you can get 3-4" diameter PVC for the main part, for the tip, you could take an old metal funnel, cut off the spout and fit it on with an adhesive, spray paint the gun metallic silver. For the lever on the gun, you can take 2 L-braces, over lap them where you have a square with a missing side---> _____
l l
l l
Before attaching them together this way, put a small 1" diameter PVC over the top. ---> _____
l_____l
l l
l l
The pipe should be painted black. Put some glue on the L-braces, quickly put a pipe over one, then put in the other L-brace where its assembled like above. Cut 2 slots in you gun that are spaced apart to fit one L-brace in each slot, Drill a hole through the sides and put an 'axle' through this, make sure it goes through the holes in the L-braces. If you followed this correctly, you should have the lever thing, and it should be able to move back and forth.
For details and other parts, look at some of the simple prop plans at http://www.gbprops.com

Good Luck! :)