You can buy in robinsons or hyper,ive seen 3 sizes here. I get the casserole mixes sent here from the uk.
I had bought one there (I cannot remember where I purchased it, now), but I did a temp test on it. You fill it halfway with water and cook for 8 hours. You check the temp of the water immediately, and if it isn't 185 degrees (85 Celsius) it isn't safe to use. Mine never would get the water hot enough, so I pitched it and gave up. Highly recommend checking the cooking temperature.