This page will provide you with a list of common sizes available at most metal suppliers. There is bound to be a supplier near you. This link is page one of four pages:
Speedy MetalsThose sizes are common to all major metal suppliers being presented by metal producers; standard stock sizes.
Some suppliers sell scrap lengths, others require you to buy a full stock length, often as long as 20 feet. I have found if you go in then talk with a salesperson, explain you only need a short length, you can look through scrap bins then pay only a per foot price.
Look through your Yellow Pages, bound to be a supplier near you.
Home Depot carries a decent variety of short lengths of small sizes, in different shapes, both steel and aluminum, mostly square tubing and round tubing along with flat, angle and rod. Have a look at Home Depot if your need is for relatively small box tubing, you might luck into rectangular tubing.
Look in the Home Depot fencing aisle for black ornamental fencing, some select sizes are available but not many, largest in fencing size is about 2 inches by 2 inches, fence post if I remember right. I recall seeing some rectangular box tubing, black powder coated.
Best bet is a local metal supply yard.
Billyray
