I'm building a language learning app based on podcasts and I used "The Podcast Index" first but, as I understand, The Podcast Index is not currently scraping Apple and many of the podcasts in their index are dead. For better or worse Apple podcasts is the "de facto" index of podcasts, so I had to build my own podcast index based on itunes data.
You can see a comparison of the different indexes (indices?) at https://podcastindustryinsights.com/