I don't know yet if it broke with the latest update but try Murgh's Vending Machines mod. It eliminates the need for a shop by having shop supplies in a vending machine. Stick one in your yard, canteen, or common room and inmates will buy shop supplies and satisfy the need that way.

One thing that helped for me was to assign at least a little bit of time for "work/free time" in the regime. The way it works is pretty simple. Two basic concepts come together for a successful shop:

1. Often you will have at least a few prisoners that neither work nor take part in any reform programs. Those prisoners change very often, so one prisoner can work on one day but then maybe the next day he decides that he doesn't want to work so his job is either left empty or filled by another prisoner.

2. Prisoners are only able to go to the shop when they are on free time

Those two things combined mean that if you assign work/free time in your regime, some prisoners will work but those who didn't find a job or didn't want to work will have time to go to the shop and buy something with the money they earned on previous days when they were working. No need for mods, it is already working normally in the game (assuming it is not bugged)