I've only tried this on the free episodes.

In firefox go to dev tools, then network. Click on the Play/Performance Analysis button. You maight have to reload the page.

The media element of the page, the .mpga file in this case, will usually be at the end of the list.

Right click on it and there are various options to take the link. You can use curl to download it or whatever works. I just copied the link into a browser address bar, deleted the characters at the end until it looked like a media link, loaded the link which started it playing, then saved the *.mpga file in the browser.