Where do you get the missing-episode audios? I've heard about them, but I don't know where they're sold.
As Philip said they're available on CD for purchase. I was fortunate enough to have a friend with them which he lent me, although some were ripped audio files (like the Colin Baker-narrated Macra Terror from cassette tape). I've never been through all of them like this before; in the past I've only ever focused on The Power of the Daleks & Mission to the Unknown (my two favourites), plus a couple of others.
Still have to acquire decent copies of Macra (which was a terrible transfer and would prefer to hear the Frzaer Hines version) and The Underwater Menace (the CD was badly scratched).