I think there's something of an assumption that if you're willing to buy a doujinshi, in Japanese, off a Japanese website that likely only ships to Japan, you're probably familiar enough with pixiv to look through the artist's works for the preview for whichever douj and find the link to Toranoana or Melon Books or whatever. And the artist's page here almost always links their pixiv (if they have one) or, failing that, it's usually on the publishing info page which is almost always left in the scans. For non-doujin manga, googling or checking baka updates manga, as Faust suggests, is trivial.
Really, if you're willing to jump through the hoops to buy JP manga, the amount of effort it would take you to find the info you're wanting is trivial in comparison.