These guys have everything you need. Have a look at other's headers for an SLC and an LS engine. Estimate the number and type of bends, 90 or 45, etc. Get a few extras of each and a length of straight tubing as well as a couple of 180's. They also have flanges, slip-on collectors, etc. Watch their clearance/B stock, sometimes they have a really good deal on a box of tubing you can play with. If you get the correct diameter and gauge you can use it later to build your headers. I used 16 gauge (.065). It's a little thicker wall so heavier than 18 (.049) but I think it is easier to weld.
I still recommend mild steel if you are new to welding tubing and have them ceramic coated. Do coating BEFORE you run them on the engine. If you want to do stainless read up on purging the inside of the tubing. Necessary for stainless tubing.