I've been trying to read the untranslated chapters of Minegishi wants to feed Otsu-kun on pixiv (https://comic.pixiv.net/works/6059). But, the chapters all say something like "The publication period for episodes 15-② to 23 has expired." I'm fine with paying, but it doesn't looks like you can purchase the chapters? I've lost all of my Japanese so I can only read hiragana and katakana now, but no vocab, so I've just been relying on google translate to navigate the website. I can't find anything that explains where I can get the chapters, or when they come out? Are there any translators out there who know how to use pixiv comic? and how to read these chapters/
If it's been taken off pixiv, then that means it's been compiled for sale. EBJ (vol. 3: https://ebookjapan.yahoo.co.jp/books/562000/A002532300/) or similar sites selling digital manga publications will have it. Most of them take foreign cards.
So, as Chiisai-hime said, you can purchase the volumes (either physical or digital) from other sources. But, if you want to buy it off pixiv, you would need and account and purchase based on 単行本(takoubon) with their coin system (it looks like its 990 coins per volume).