My experience of 16ga ammo availability differs form those talking about scarcity. I can find the excellant 16ga Fiocchi Golden Pheasants loads in any sporting goods store when I am on the road hunting pheasants and grouse. The bigger sporting goods store have a wide range of 16ga ammo to chose from, as well. I've even been able to find 16ga at gas stations.

That being said, I am very particular about what ammo I feed my guns when hunting regardless of gauge, and thus always buy exactly what I want on the interenet and bring it along. Just like gas for my car, with a little planning I've never come close to running out.