Around here we find it at local oil distributors. I guessed they only had farm and truck stuff but they stock a case or two of aviation products. If not in stock they can get it quickly. Will save a lot on shipping fees which offset any internet savings.
I bought a case here: http://www.oil-store.com/ and I will definitely be buying from them again. Great prices, easy to use on-line store, and super fast standard delivery. I got it in 2 days! Look for sales if you can wait.