Too bad you are not up in the SF Bay Area, Keeble & Shuchat has some. I just picked up some 100-ISO cartridges, and the guy asked if I wanted any 400-ISO cartidges and pulled out a box of at least 24.
If you can't find any, call or email K&S. firstname.lastname@example.org
If you call, ask for the darkroom dept. Be patient, things are a little confusing right now, they are moving the darkroom dept from one building to another, and stuff is in boxes to be moved to the other building. So the guy answering the phone may in the other building from where the cartridges are.
B&H, Adorama, and Freestyle have them and you can get used ones that had film in them and left some film out. Tape the new film to the old film end coming out of the cassette and reload. Many cameras that have DX also have manual ISO so you could just use the plain ones.
Or you could try making your own DX labels. Recently, I'd picked up an excellent condition Pentax P30t for cheap, but then discovered it required DX coded cartridges. After browsing through the web for DX label solutions, I'd decided to make my own using materials that I already had at home: aluminum foil, double-sided tape, and masking tape.