Cassettes aren't dead, but there are only a handful of manufacturers left putting out cassette pancakes. I wouldn't be surprised if at least a few of the brands mentioned here are actually using the exact same tape loaded into a shell with their name stamped on it.
Your best bet is to find a duplication house that sells bulk blank cassettes, as Kurt mentioned. I've always favored BASF tape stock in a shape shell as well. Flip through the classifieds at the back of any recording mag and you'll be sure to find a cassette duplication house that'll ship you some blanks. If you go this route, be sure to specify shape shells, as they have a pressure pad assembly that does a much better job of maintaining proper alignment in the tape path than a regular shell. Also try to keep the length to a minimum (c-62's, +/- a little bit, maximum). Cassette tape stock comes in varying thicknesses to accomodate longer loads. A c-64 will be loaded with thicker tape stock than a c-100. The thicker the tape, the hotter you can push it. Not to mention the fact that thick tape stock is physically sturdier than thin stock, meaning less chance of it wrapping around a capstan or getting damaged some other way.
I can name at least one place to get these if you PM me. Hope that's not against the rules here..........