Does anyone know a good website I can learn Russian cases?

This site (http://en.glosbe.com/ru/en/вода) is not exactly for learning Russian cases, but i use it for checking them: if you enter a Russian word and click on "show declension", you will have its declensions with the proper accentuation (useful for mobile accents). If you're looking for the tables, I think the wiki is quite complete: https://en.wikipedia.org/wiki/Russian_declension#Nouns

Also, you have this website which is very complete and clear: http://russian.cornell.edu/grammar/subject.htm