Hundreds of new podcasts are added on a daily basis.
If you don't find a podcast you can try to switch to iTunes database. To do so, just check the iTunes checkbox in the search engine screen. iTunes database indexes more than 250K podcasts so you should find it there.
If you don't, you can still subscribe to this podcast if you have it's RSS feed url. Just press the + button to add a new podcast then select the 'Add RSS feed' icon and paste the podcast rss feed url.