There are two major chain supermarkets in Volcán: Romero (a Panamanian chain) and Bérard (a French Swiss chain). Both are dependable, with a lot of variety. The latter tends to be more geared toward the gringos, and the prices reflect that, though prices are generally low; one person can eat nicely for about $50 a month. Local fresh produce is always excellent. You can also get most U.S. brands of your favorite things, but they are of course imported items, so they will cost you more.
My suggestion is - you live in Panamá, eat like a Panamanian: the food is healthier, fresher, and cheaper! I must add that you can also get extremely fresh produce at any of several fine farm stands. I buy my fish directly from the fisherman, who comes up to Paso Ancho once a week, driving around tooting his horn - you will never have fresher ocean fish unless you go catch it yourself.