For my ODST undersuit, I used army surplus black thermal underwear. I would have gotten both in the weaved cotton version, but the bottoms didn't fit, so I got those in Polypropylene. (Probably a good thing; it's very good at maintaining a good temperature) Overall cost was around $30, because the polypro bottoms were expensive. If you get both in cotton, it'll be half that price, and actually look more accurate. (There's a waffle pattern on the suit, if you look at it closely)
From there, I followed Sean Bradley's guide to the various appliques on parts of the undersuit. Look up Sean Bradley. I can't remember where I put his PDF.