Check out the LodesTone Media page, which has lots of Firesign Theatre stuff, and is supporting the Firesign Theatre in their online manifestations.
(Click here for search options)
There is also a newsgroup called "alt.fan.firesign-theatre", but it is not used much because the "firesign-theatre" string is longer than 14 characters and so it breaks some of the newsreader software.
Many of their old albums are still available on CD, and are put out by Mobile Fidelity
For those familiar with the Firesign Theatre stories, an online Multi-User Shell (MUSH) is available to play around with; log-in with the name "Regnad" or "Regnad Kcin" (case sensitive). Be sure to type "home" and "QUIT" when you are done!
Firesign Theatre Sounds are available in WAV format from Richard Arnold, as well as .AU format, compliments of Josh Hayes.