It depends on what all services they are publishing their podcasts. Spotify may be one service, but try to look for their websites and see where else they publish. If they’re on iTunes, Rhythmbox may be able to find them (at least that’s what your screenshot suggests). Antennapod basically does this—it looks at a number of publicly accessible services where podcasts are published. So yes, if you get the feed url from Antennapod etc. you should be able to use them in Rhythmbox (or any other app).
Found the services Antennapod looks at, so start by searching in these?
Here’s your last one on podcast index:
Edit: just tested and works on Rhythmbox—found the URL and then subscribed without issues:
