My local autozones always seem to have about 8-12 quarts every two weeks at the least. They have orders come in about once or twice a week, so I think I have a good supply handy. If they are out, usually you can ask the stock manager if he can put in an order for more of it.
Also, autozone seems to run promotions on castrol alot (at least the ones here on the west coast), and that's when they have a lot of castrol GC on hand, like 24 qts at a time, in boxes and not loose bottles. I have about 7.5 qts of GC on hand, plus some chevron and mobil; maybe I'll go and buy more soon to stock up.