I have purchased it at AutoZone.
If they do not have it then ORDER a case...
It will usually arrive at the store in 3-4 days unless you live in the sticks...
Simple as that.

Regarding using a heavier oil...

I would shy away from that at all costs. The engine is manufactured with very close tolerances and as such the thin oil is required to wet moving parts, main bearings and such.

About 3 months ago, I called in this question to National Public Radio's talk show "CAR TALK".
Click and Clack were quite emphatic that if an engine calls for the new 0W-40 that heavier oils should not be used unless the owners manual specifies such.