Also available on GOG: https://www.gog.com/game/tis100 if you don't like adding DRM to a DRM-free game by purchasing through Steam.
I bought TIS-100 through Steam years ago, only had it a few days before they decided to suspend my account for 2 weeks and take away all the games I purchased. They we're all on sale when I bought them originally but the sales were over by the time I got my account back. I finally got TIS-100 back last week through GOG because I happened to see it and it was half-off (would have bought it anyway). Can't wait to finish it.