cdnow.com has cassettes, I believe. Although the selection isn't too compelling. The Coconuts Music stores near me have a decent selection of cassettes, esp Jazz (at least my store does), but the prices are a little high, although nowhere near as high as the CDs. They really discount the classical CD/tapes, though. (Since they can't move that product like those $18 Pop music Cds.) :-/
Also most K-marts tend to have fair cassette selection. YMMV.
Also most K-marts tend to have fair cassette selection. YMMV.