How can I redownload an episode of podcast which I accidentaly deleted?
If you are in "album" view, there should still be an "album" placeholder for your podcast, even if you do not have any more episodes stored locally in your computer. Right click (or Command click) on the podcast's album/icon and then select the option "Show All Available Episodes". This re-downloads the entire list of episodes in the podcast's RSS feed. 

Is important to note that "Show All Available Episodes" does not re-download the episodes themselves, but rather the list of episodes. This is the same list that anybody sees in iTunes if they haven't subscribed to the podcast yet. You will be able to select and re-download individual episodes from that list that you have already downloaded and deleted. Hope this helps.