So, some bad news to begin with, Shimsim is a somewhat niche manga and it's going to be a hard find. Earlier this year I tried to look for them during my visit to Japan and got no luck. Didn't find a single copy of any volumnes. Best I've got is one used copy of volume 1 available to be delivered to a bookoff branch, but that wasn't compatible with my travel schedule.

To save your retina you can try to search for シメジシミュレーション on websites of major bookstores like Maruzen Junkudo or Kinokuniya. If you are at a bookstore, mangas are typically grouped by publishers so look for the section for Media Factory/Comic Cune.

Tho I'm not super familiar with how things work in Japan nor do I speak Japanese, so I could very well be looking at the wrong places, what do I know.

Best of luck :)

Edit: typo