Sounds like a cool project! You'll want to get a solar panel that can charge a battery, which can then power your heater. The wattage you need will depend on how much power your heater uses and for how long. You can find calculators online to help you figure this out.
For buying the solar panel, I recommend checking out this page for a good selection of panels. Be sure to read the reviews and product specs to make sure you're getting the right one for your needs. Also, don't forget to get a charge controller for your battery, which will regulate the charge from the solar panel and prevent overcharging. You can find these on Amazon as well.