Well thank you all for your responses. I've put all of them to good use. I even had an audiogoner contact me directly with his stash of unused good tape.
Ktrogers: I gave eBay another try. Last time it seemed hawkers were treating sealed cassettes from the '80s as ancient artifacts worthy to start bidding on as an heirloom for $30+. But if you keep looking you find some people who are offering NOS tape at very reasonable prices. I picked up a few there. Amazon is a better source it seems.
Johnz: I in fact ordered a couple SM-60 tapes to try, they seem like they're rebadged TDK SA from the '70s but I'll hear how they record shortly. Much better than Maxell UR, but probably on par with XLII? BTW I can still pick up a 5-pack of XLII anywhere for ~$7.50 which is a very good deal. Even if it isn't the world's greatest tape it still sounds more than acceptable with a good deck. I doubt the formulation has gone unchanged since the 70s! Maxell customer service had no clue.
Blueranger: Actually my last stash of tape was a case of Maxell MX-S (their top metal tape) from a locally-owned electronics shop at their cost in the small city I live in. Little did I know then that was the last run of MX-S, I thought the store had just abandoned them. In future travels I'll stop in small town stores and hope to be as lucky.
Mosin: I have been in touch with Alan who is shipping me several Sony UX Pro to see how they sound, with the option of buying many more. Fairly priced and a good source.
So it seems like cassettes are like vacuum tubes, the best are no longer made (ie no metal tapes anymore) though you can still buy adequate stuff new (ie Electro-Harmonix tubes). But you generally have to pay dearly for the best NOS tape as it slowly disappears.
Unlike the increasing demand for tubes, there is certainly a diminishing demand for good cassette tape, but I stand by my claim that a good tape in a final-gen high-end cassette deck is a more flexible recording media than anything except perhaps DAT or MD, and arguably better-sounding.