Depending on your location-
There are several good sources in Grand Rapids, Otsego and Kalamazoo.
Dan's Sport & Ammo in GR on West River is a good source for powder, bullets and primers. Its a small shop (actually a closet in the back of a radiator shop) and their hours are only during the day. I've never acually been there- my wife picks up stuff for me.
Schanz Supply in Otsego has a great selection of materials, but the service is iffy at best.
I personally like On Target in Kalamazoo. Not a huge selection, but stocked in depth for popular calibers, good selection of powders & primers, as well as die sets. Also a bazillion guns to fondle, and great service.