Your best bet is to go with 6061.
Some grades are easier to machine than others, and 6061 is best (or at least best of the easily sourced grades)
You will find info on the various kinds and their machinability on the wiki
Definitely checkout Winston’s video for recommended feeds and speeds:
As to where to buy the aluminium from, I will be of no help since I live on the wrong side of the pond to answer that question, but the American folks here will be able to provide good tips I’m sure