yeah it's actually really good but I don't think there's a "paper" version per se at all.
The guy literally made like one chapter as a test of software and like the upload service he was making, and it got some buzz so he made a few more then it got a fanbase. like 70-80 chapters in a publisher hooked him up with a real artist to basically give it a real go and it got an anime.
So basically there's two comics but even the latter one is digital only I think. They usually go by One Punch Man (one) and One-Punch Man.
The original artist/ writer is named one, so it's pretty confusing.