Why don't you want to use flexwatt? It's the most reliable thing you can get IMO. You can buy one of the pet store UTHs, but you NEED a thermostat (or at the very least a dimmer) to control it. Any heat pad is going to get too warm without something to regulate it.

Even if they seem ok, corn snakes really do much better with an 85-88 degree basking spot. It makes digesting easier on them and boosts their immune system.