Zoo-Med and Exoterra both make excellent models, but there are many others. Ideally, you want to heat 1/4-1/2 of the enclosure with the UTH. I'd recomend using a rheostat to control the temperature as opposed to depth of the bedding. Both methods work, but the rheostat is safer. Lack of education amongst pet-store employees is upsetting. It doesn't make a good impression on new herpers.
Only a few of the larger colubrids (the large family of snakes that includes rats, corns, kings, gophers, garters, racers, ground snakes, etc.) intentionally prey upon invertebrates. These include the racers, whipsnakes, and greensnakes. These animals are sight oriented predators that cue on movement. Corns, rats, milks, kings, ect. typically rely more heavily on olfactory perception when searching for food. While snakes in this last group (collectively termed the "Lampropelteni"
may eat an insect on occasion, one would be hard pressed to find evidence that this is a regular occurrence. These snakes generally feed on rodents, lizards, frogs, and birds. They are vertebrate feeding specialists, as evidenced by thier constricting behavior. Smaller species of colubrid (such as ground snakes and sand snakes) often specialize in feeding on inverts.
-Cole