For manga, I’ve always purchased from Barnes & Noble and Books a Million, mainly because they’re near my house and I’m more of an in-store shopper, but they’ve got solid online presences and a satisfying catalogue. Plus, both have pretty good membership reward programs (B&N in particular is $40/yr and gives you 10% off pretty much everything, free shipping, and a $5 reward for every cumulative $100 spent, plus some other stuff).

And if those places don’t have what I’m looking for, there’s always Amazon

Germany: Egmont Ehapa Manga publishing

I use https://www.amiami.com/eng/, https://jlist.com/, amazon and eBay for merch.

https://jastusa.com/ is good for games.

J-novel.club have drm free LN and mangas