I try to buy metal and plastic supplies locally, partly because it is good to have places you can go to and get something immediately, and partly because I like see local jobs, so Boise doesn't turn into a ghost town. (There are parts of Boise along State Street that are beginning to get a bit scary in this regard.) Another advantage of buying locally is that unless you are ordering a lot of stuff, the shipping costs for a foot or two of aluminum tubing will quickly add up to a lot more than the material itself. I buy acetal from Interstate Plastics, and I buy aluminum from Gem State Metals. But that assumes that they have what I need.
I needed some 2.75" OD, .125" wall round aluminum tubing, and some square tubing of various dimensions for the fixtures to let me clamp round stock into the chop saw. But Gem State Metals did not have any of the sizes that I needed. Speedy Metals, however, did--at such a reasonable price that it probably came out with shipping close to what I would paid someone locally. And I am getting all the parts cut to size--one less of cutting operations to perform on the chop saw, which scares the wits out of my wife when my son is using it, and provokes a healthy respect in me when I am using it.