Depends. There are two main methods, both of which split into several other branches.
If you prefer to go physical, the most straight forward (albeit expensive) method is ordering straight from Japan, this is usually done either through CDJapan, amazon japan, or MangaRepublic. All of these have their pros and cons, seeing what works would be the best here. Also there are certainly local options as well, depending on where you live, like Kinokuniya.
If you don’t mind reading digitally (most use this method), then the current main options are Bookwalker, amazon jp again, but digitally, and Kobo. These also have their pros and cons (in particular, bookwalker is made so you can freely order even as a foreigner, while amazon might ask you once to prove, that you are japanese, which a simple vpn can solve).
Whichever style you go with is your preferance. In general physical manga is higher quality, but digital manga is easier to search through, especially with tools like Mokuro.
The home thread for each book club almost always contains buying options, check that out if you want examples.