Large diametre Aluminium Rod

[9W Monighan]

Try Clinton Aluminum in Ohio USA

Order flatbar the desired thickness then, You may be able to bandsaw cut your round from plate slightly oversize then turn in your lathe.

I miss read your post.
You want large quantity. Check with supplier I mentioned. I don't know where my extrusion suppliers get their billets.
But that may be worth checking into. They buy rounds in various diameters to use in their presses.