Best thing is to go with two sources....one for supplies e.g. Enco or even McMaster-Carr, others are MSC, Travers. The other for supplies, that is if your looking for aluminum, brass or stainless. I've had real good results with Online Metals and a couple others. Do an internet search.
If you're only requiring small pieces, say a one-foot piece of 6061, 1x4 or 12 x 12 x 1/2, then I'd stick with McMaster. The quality is good and they're really fast.