Doug,
I can't read Cardew's awful Stockhausen rant. I also don't care for any of the music by Cardew in his Maoist phase. Just more proof that good politics does not necessarily lead to good art.
For Cardew's avant-garde period works, there's a recording of "The Great Learning" by the Scratch Orchestra on Deutsche Grammophon (this was when Brian Eno was in the Orchestra, BTW). This is widely available.
There is also a nice recording by John Tilbury of Cardew's piano pieces available on Matchless Records www.matchlessrecordings.com
You could also get one of the 60s era AMM recordings with Cardew in the band, either AMMMusic or "The Crypt", both are in print and widely available.
It's also worth investigating anything by the musicians in Cardew's immediate circle. The improvisation group AMM (as someone once described it, "John Cage Jazz"), or any of the solo recordings by Keith Rowe. If you like more classical recordings, go for the John Tilbury set of Morton Feldman piano works.