Since Episode 2 it was clear to me: I need this costume. I bought it in Ebay but sold it meanwhile again.
Clonetrooper
Just like you use on the Stormtrooper a black shirt and black leggins, this also fits for the Clonetrooper. Actually the suit underneath is ribbed like Darth Vaders suit, So you may want to get it tailered for you
There are two possibilities. Either you buy a cheap canadian Kellogs-promotional helmet (very thin material) or the much better rubies Helmet.
Compare yourself: Left: Rubies, right: Kelloggs
I just used the same ones I used with my Stormtrooper costume
Here I cannot be much of a help because I bought the costume at ebay and since then I lost contact to the seller. Perhaps from time to time a costume may show up in ebay if you keep searching.
It took me only 4 hours. to build a blaster out of wooden parts and a plastic-tube. Find some blueprints and be creative. Sometimes you find replicas on ebay, too.