Today the world is your oyster, i.e. it's as easy to order something from the other side of the globe as it is to god down to your nearest supermarket, so if something isn't available locally you can almost always find it somewhere else!
I try to do 50% of my shopping at a "local" hobby store to support him and the rest online. The "local" guy is more expensive but I have the order next day if I order before 10 o' clock! Online orders often take 4-6 weeks to arrive...
If something is truly out of stock then I try something else or mix my own, if possible.
I also keep stock at home of the more common paints I use and I seldom have less than 4 unopened jars of XF-69 for example because that's what I use for primer!