Historically, I have used
Azusa Engineering for all my Aluminum sprocket needs (especial largish ones). They make great aluminum sprockets.
Azusa does not sell to the public. In order to get them you have to find an Azusa dealer to order them for you. I have had very good luck with a local company here in Michigan,
Recreational Leisure, Inc. They have gotten used to the idea that FIRST folks are in a hurry and make allowances (For example if you ask, they have Azusa ship them directly to you rather than to them to ship to you). By the way, Azusa carries almost zero stock. They cut your sprockets when they get your order.
There are other sources as well.
I have not followed the links, but InnovationFirst.com, and AndyMark.biz both have "alluminum spockets" listed on their websites. Check them out.
Joe J.