You can get circular tubing at home depot no problem. It's getting square tubing that is hard. If you want square tubing, fucking go to a construction site and steal some, that's what I did. As for making the actual rail, go find a base, (like a 1 cm thick piece of light metal) that has good stability, fucking plan if you want a straight/kinked/bent-however-you-fucking-like rail, welds some supports, weld the base, sand the top of rail (where you will be sliding), buff that shit, wax it, voila... you have a rail.