There are a number of text-to-speech (transcription) services out there. Kalzumeus uses CastingWords. The cheapest option (price determines speed of transcription) runs a dollar a minute, so it'll be pricey for most podcasts--but you could consider it a donation and send the finished transcript on to the podcast creator, ideally convincing them to start producing their own transcripts.

There's also a lot of transcription software out there, but the quality might not be as good as you'd like.