One place you can find them are here, here, and here respectively. I don’t have any meaningful experience with apple products, but with bookwalker you can click the book cover for a preview that opens in the browser app, so you can see if that’s acceptable for your needs that way. If you’ll be reading in a place with a steady internet connection, and don’t have issues with the browser app, then bookwalker doesn’t require installing anything special.
I found the sign-up easy and have never had any issues like needing a Japanese address or credit card or anything like that (I think I remember maybe having to enter some information in half-width alphanumeric, but otherwise super convenient and easy), so I’d definitely recommend it as a surprisingly easy way to get an extremely large amount of Japanese ebooks. You also might be interested in the Freebies thread where information about periodically free manga gets posted (I’ve seen the first few volumes of Parasyte go free-to-keep a few times this year so it might be a semi-regular occurance).
(there’s other, similarly easy alternatives out there, like Booklive, but as far I’ve seen the differences between them are pretty minor.)