Five terms get used for adult comics, and they are not synonyms. Each names a different kind of book and a different place to buy it.
Porn comics are sexually explicit comics. The legal sources are studio sites such as Slipshine, Filthy Figments, Kirtu and Affect3D, the FAKKU store for translated hentai, and artists selling on itch.io. Prices run from a free sampler to about $100 a year for a full archive.
Adult comics, in the narrower sense, are mature-rated stories for readers over 18. GlobalComix, WEBTOON and INKR carry that shelf, and GlobalComix states that explicitly sexual stories are not accepted. The explicit shelf sits elsewhere.
NSFW comics are webcomics that artists publish on their own sites, usually free to read. Patreon, SubscribeStar, itch.io, Kickstarter and print books pay for them. Oglaf, a weekly fantasy sex comedy with 2,845 paid Patreon members, shows the model.
Erotic comics are comics where the sex belongs to a longer story. Iron Circus sells erotic graphic novels such as Yes, Roya as print books ($15.00) and DRM-free PDFs ($10.00).
3D comics are built from posed 3D models and renders instead of drawn pages. Affect3D sells them title by title as DRM-free downloads from about 200 artists, and one comic costs $12.50.
To choose a place to read, start with the best porn comics sites, then the lists for NSFW webcomics, erotic graphic novels and mature-rated and explicit stores. Japanese work has its own section under manga.