So it seems that Digimon S1 to S4 have been removed from iTunes recently as well as Crunchyroll. It is unclear if it is down to licensing agreement with New Video Group expiring or not and it is unclear if this will effect the DVDs. At the moement you can currently buy Data Squad (S5) and Fusion still.
Saying that if you purchased any of the Digimon seasons that have been remove you are sill able to redownload them the cloud.
Hopefully they will reappear again soon in the future but as of now you have limited options to watch the Digimon English Dub.