Add to above, many of the Podcasts (all?) in iTunes are MP3s, so, just need to navigate to your library in Finder and copy over.
"All mp3" as I just did a search on my library and only one was m4a, but rest in that show are all mp3. And seem to recall that publishing a podcast in iTunes requires the author to handle file storage of the file(s). So as above, authors usually have a website where they are hosting their podcasts from, and probably in mp3 format.