I assume you are talking about physical copies? Disclaimer that my answer is US-centric since that is my location and mostly focused on manga. Also, it is unlikely to be particularly helpful.

I almost exclusively consume things digitally these days, so my primary (legal, when available) source is the official publisher/streamer. That means places like amazon for kindle versions of things, bookwalker, etc. Also, I can’t remember ever buying physical copies of anime any time recently. If I can’t access it via legal streaming, I find other places to get it on the internet. In the before times (pre-streaming), conventions used to be the best place to find DVDs of anime, but I haven’t been to a convention in a long time, so I don’t know how true that still is.

For physical manga, my go-to back when I bought it was a local comic book shop. They had a decent selection of stuff, but they would also buy things on request. This was particularly helpful if they had most of the volumes of a series, but with a couple gaps. I used to walk by the shop on the way to the subway every day on my commute, and didn’t really think anything of it since I don’t really read Western comics. However, one day they had a bunch of Cardcaptor Sakura omnibus volumes on display in the window. That is what led me to find out they carried manga as well. However, two years ago I moved and am not nearby that store any more. The only physical things I have bought since, I got from amazon (TEIS LNs).

That being said, other than amazon (which will almost definitely have most of what you are looking for), there is gomanga which sells physical manga/LNs (they are the official store of Seven Seas). Barnes and Noble also has a pretty good manga selection. My last pick is pretty local, but if you are in the greater Boston area, Newbury Comics has a good selection as well with some convenient locations (even though their website is pretty bad).