Sam's Wholesale Club if you have those up there may have it
Thing about Aeroshell, not just anybody can get it from Shell. There's only [IIRC] 50 distributors in the country, or one per state. Everyone else must buy it from them and resell it
You don't really HAVE to use aviation oil in a 220. I don't see any reason why a 15w40 diesel oil wouldn't work great, especially synthetic. I also know a few people that use 50w Valvoline VR1 in them with no problems.
If you do this, I would recommend running an external spin on filter adapter that can easily be plumbed into your oil cooler lines. The filter will catch the junk that the auto oils' superior detergents will clean out of your engine