Is there a website where one might find a list or dictionary or difference dialect differences, -not just kansai- or "slang" of certain regions or prefectures?
I have found a couple of links for your reference:
This appears to hold a list of all dialects in Nihongo:
Hope this helps.
As ssb already commented on your question on main, you're looking for 方言辞典 "dialect dictionaries".
The two online sources I know are