The only one I ever looked for was Sam Harris’s podcast. All I did was search “making sense with Sam Harris RSS”, and one of the top results was the official RSS feed lol. I was looking for a pirate feed but found the official. Which if you can’t afford it you can email and get for free so I don’t think he really cares of its that easy to find.
TLDR: try searching podcast name + RSS and maybe you’ll get lucky.