You might try Gander Mountain. I really don't like to recommend them over a LGS but the last time I was looking for some accessories they had a fairly good selection. You might try calling Bradis as well, as I know they too have a section devoted to muzzleloaders.
The mother-lode of black powder shooting supplies is Dixie Gun Works. For $5 they will send you a 1 1/4 inch thick *printed* catalog of everything you never knew you needed for your black powder firearm
I bought all of my muzzleloading accessories from Track of the Wolf, except for the black powder itself, which I got from Grafs (as they allow you to make a much smaller minimum purchase than Track).
I was in Gander yesterday and they wanted $27 for a pound of Pyrodex "P". $15 mail porder, but requires hazmat. Dixie, buffalo, RRArms and Possibles are pretty good on line vendors for flasks, measures, sabots, round ball, patches, wads, etc. I have never personally tried track of the wolf, but they look like they have a pretty good selection.
The supplies are pretty thin right now at local Gun Shops. You usually see a lot more muzzle loader supplies in the fall getting ready for deer season.