Is this alloy readily available to shops in the U.S. ?

I do not know if you've received a reply yet but this is a very common alloy steel.

it is not readily available in canada would it be easy to find 8.5 " dia. by 4" thick?

Yes,

That is available. It's 197 pounds/foot. It you need something made, let me know.

Cary Hutchinson
caryhutchinson@hotmail.com

6150 is a chrome/vanadium alloy. Fairly easy to mill. I got some from Metals Express, no problem.