There are A LOT of manga on this planet, main-stream manga, non-popular series, obscure series, and doujinshi / self-published works out there (which I think are super rare), I’m glad that in the Americas there are various stores where we can buy manga.
The only downside to these stores are their contents, B&N (Barnes & Noble) only has mainstream and semi-popular series, for example: Dragon Ball, DBZ, HxH, Chainsaw Man, Jujutsu Kaisen, I’ve also seen the FLCL Omnibus once. Thankfully Book Off also has a lot of other series such as: Ranma 1/2, PTSD radio, Homunculus, and Toriko.
The real deal is when scanlators come in. Mangadex is the best site to find scanlated (scanned and translated) manga, which is also controlled by the community, plus it looks super elegant. I have seen some series which I’m thankful are on this website.