I suppose by sources you mean texts, books where you can read about it?
Unfortunately I don’t know of any books in English that deal with this matter, and aside from Wikipedia there’s not much about it in English on the internet as well. I suggest you browse ‘slavic mythology’ and read everything that seems quality, althogh there isn’t much. If you speak some of the Slavic languages this will be much easier, and you will find much more stuff.
Here is a pretty good site that is in proces of translating to English
and you’ll probably find good stuff here