Bob
Here are a couple of sites that have a selection and some prices with shipping. Structural is usually erw welded and dom is drawn over a mandrel and no seam.The dom tube carbon steel is usually the choice over structural.Tigdepot has a selection of tig rod with explanation of application,very helpfull.
Al
http://www.tigdepot.com/
WWW.twmetals.com
http://www.metalsdepot.com/index.phtml