So I have now used two sources in the area. Boulevard Sales (http://boulevardsales.us/
) has a decent supply, it's definitely an old-school place and their prices are a bit on the high end. The second is BMG Metals (http://www.bmgmetals.com/
) which is a MUCH larger shop and has all kinds of stuff. There are a few things they're more expensive than Boulevard on, but in general they've seemed to be cheaper. I normally call both and get quotes over the phone and then decide on price. If you do contact them, make sure you ask for total price, including any cutting fees, vs. just the per-foot cost.
Boulevard does NOT have DOM tubing, BMG apparently can get it but I have't tried yet - been mostly dealing with square stock.