Do you mean 75W90?
Lordco may have it in stock. Otherwise, you will have to ask them to order it in or contact a local dealer/reseller which
can be a pain in the ass. I may be wrong but I recall Jim @
Overseas Auto being able to get Amsoil. Call to be sure.
Your next best bet is
Red Line 75W90 NS Gear Oil. It meets the same specifications as the Amsoil you listed and is fully synthetic. Not to mention, it has the EP additives and meets the GL-5 requirement, similar to the Amsoil product. Product availability is widely available.
Driver's Edge has it for $12.95/quart. Make sure you get the
NS (Non-Slip.)
If you're dead set on getting the Amsoil, stop by Trev Deeley Harley-Davidson on Boundary Road. Pretty certain they will carry it.