Sphaerocerid flies or drain flies likely! Both like wet and damp and dark, as well as organic material that is going bad.

These little guys are determined, so a change of substrate is needed at this point, as they are already using it as a breeding ground. I would wipe out the tank/cage thoroughly as well, before throwing in any more. In addition, you need to kill the adults before they adopt any new substrate you put in. Put out bowls of vinegar with a tiny bit of dish soap mixed in around your tank. The vinegar will attract any adults and the soap will drown them. Change the bowls every two or three days and keep them out til the adults are gone.

I wouldn't blame your reptile shop, by the way. These flies probably came from the drains in you house, or outside.