I just got back from Lowes. There isn't any info there as to which alloy the aluminum is.
If it were me, I'd get my metal from a local metal supply shop. Most of them have a minimum to buy (usually around $50). I'd make a list and get several things at once to meet the minimum. The metal supply places will be cheaper than Lowes as well. Definitely call a few places, everybody's prices are different.
I haven't ran into too many customer friendly metal vendors; seems to be normal. If they're a jerk, don't worry, its not you.
