If you can't find something on Pottermore, your next bet is the Harry Potter Wikipedia. It isn't as reliable as Pottermore since it isn't run by J.K. herself and since anyone can edit it, but it's still your next best bet. It's a great page, and it has tons of wonderful information that can really help your fanfiction fit canon. I even love browsing it just for fun, as some of the material is put in there in fun ways, and quite a lot of it is interesting.
I realize that you cannot click the link, but you can copy and paste it into search, so here it is: http://harrypotter.wikia.com/wiki/Main_Page.
Also, if you are into other fandoms, you can find other great information for those ones too!
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How To Write: Harry Potter Fanfictions
FanfictionA simple guide to help you on your journey of writing a Harry Potter Fanfiction. [started; December 22]