Wow, that is a hard one. First as cassettes become more unavailable, you can find close-outs at rediculously great prices, and folks charging 2-3x the list price for the same tapes. Low bias tapes are still available most everywhere at reasonable prices. High bias tapes are harder to find, Radio Shack seems to be the only reliable source, otherwise it is catch as catch can. What seems to be currently manufactured is TDK SA, TDK SM (same tape different cassette housing), Maxell XLII, and the Radio Shack stuff. That applies to the USA, I am sure that more is available elsewhere, here if it is not a billion dollar a year business the corporations just drop the product:thumbsdn: .
It used to be advised that you stick to the 60 minute cassettes because they had a heavier tape base, but my understanding is that now with the reduced market they no longer even manufacture the heavy based stuff and just load 60 minute cassettes with the same tape they use in the 90-100 minute cassettes (Anyone got better info on this?).
So, a lot of words that do not actually give you the answer you were looking for. The best advice I can tell you is, find a good deal and stock up on the stuff.