Their site is super easy to check for sizing and wall thickness of SQ, Round, and Rectangular tube. Save it in your favorites for future refference. When you open the link, just click on the desired tube shape and the sizes of what they stock are there. Of course it's going to be different from YOUR steel tube manufacturer but its a great quick refference for info. This is where I bought all the DOM tube for my cage at a GREAT price for USA made DOM.
http://www.tottentubes.com/round_tubing.html
http://www.tottentubes.com/round_tubing.html