Tim Wescott wrote:

I would really, really like to get my hands on some 3/16" x 1/2" aluminum
bar -- 2024 would be nice, but 6061 would be acceptable. 3/16 x 3/8
would be acceptable, too, but if I had to have one it would be 3/16 x 3/8.
There are a bunch of online sellers that are happy to deal in smaller
amounts. Online Metals, Shapiro Supply (local to me) and several others
have quite a wide range of alloys and shapes available.
Just do a Google search on "aluminum bar" and you'll have pages of sellers
to choose from. Several have the exact size you are looking for.

Jon