it's in the shops! you go up when you clear a room and progress, there are 3 locations!
Checked it, I haven't finished the first room before because I thought that's the only one. Really fun, good job!
The only thing I didn't like was starting from the beginning over and over again after you die, and maybe I got more powerful after every death, but finishing the rooms didn't feel faster
Changed my rating to a more positive one, Well done!