Lonnie (the Brutus guy) used 1/8" wall stainless square tubing because he was able to get a real bargain on it. His use of that tubing made it really popular, but I really think that a lot of you guys are wasting your money trying to copy him exactly.
.030" wall tubing is adequate for most brewing applications. Even .060" is overkill, but it's sometimes cheaper. Mild steel makes a fine brew stand. Sure, you have to paint or powder coat it, but it's usually 1/4 the cost.
However, if you have money to throw at a project, stainless sure is pretty.
Local suppliers are the way to go. Look up welding/fab shops, salvage yards, pipe supply companies, and steel supply companies. They're not hard to find.