I got mine at Wally World. Spring is here, so they should be getting some in.
Recently, I made one from scratch. I used 1" foam board and cut the pieces to size with a box cutter. Then, I used REAL duct tape (aluminum backed) to put it together.
Right now, it is holding a light lager at 53*F, steady, just using a frozen 2 liter bottle of water.