You can purchase waterproof heat mats that are made specifically for supplying bottom heat to plants and seed flats.

Alternatively, you can look for existing heat sources around your home. For example, the top of your fridge, light rack, or hot water heater.

Another alternate heat source around the house that some have is the top of an aquarium. The top of my spouses guppy tanks are a constant 74° with the lights off, quite a bit higher with some of the lights on.

Try the top of your computer or monitor. Those are nice heat makers.

What kind of seeds are you germinating? I might have better advice.