1.5"x12"x12" billet of 6061-T651 will run you about $75, and can be found at places like Yarde Metals
Drop Zone. They have some of the best prices around for random metal parts. (Also, make sure to buy a piece that costs as least $75 (or add other random parts to order to go over $75), or you'll be whacked with an $18 service charge).
As RC said, any machine shop with a CNC mill can bolt this to a fixture plate and mill it into a disc.