When you go shopping for SS tubing you want to use .035" or .049" wall thickness, less than that is difficult/impossible to bend without a special bender. The welded, mill finish type 304 or 316 SS tubing is what you are looking for, seamless tubing is overkill as interior finish is the same. Watch Ebay for 400 series benders by Rigid Tool, or Imperial tube benders, these are the lowest cost, if you have too much cash then go for the Swagelok benders. If you check with a plumbing supply company they may have SS tubing in stock, it will come in a 20' length so take a tube cutter when you pick it up. The "El Cheapo" tube cutters from HF will not last long with SS tube so look for a Reed,Rigid Tool, or Swagelok 308 tube cutter with SS blade if you plan to do much.
As I have instructed the new hires that have not bent tubing before, I remind them that if finished work is in left hand use the "L" mark, if the finished work is in right hand use "R" mark, for a 45 it does not matter which direction it was measured from. Last item is there is a "0" mark on both the arm and the center mandrel, line these up before you start the bend with center mark on appropriate spot, this prevents a shift in the final dimension between bends.