Where can I find or buy, in bulk, recordings of 20,000+ English words, read by native speakers? (preferably available as individual audio files with an index)
I don't want to use a "text to speech" program if possible because the quality isn't good enough for people who are trying to learn a language.
The Linguistic Data Consortium sells these kinds of data. Check out the American English Spoken Lexicon, for example.
Just to let people know what I did in the end:
I found Shtooka, which is a free program for recording lists of words easily and automatically. I will now pay an actor to record the exact lists I want.