While there is not a specific heater that Back to the Roots endorses officially, around the office we use the Tetra HT Submersible 50 Watt heater.  Native to Southeast Asia, bettas are tropical fish and are happiest in water between 75-82 °F (24-28 °C), so you'll definitely want to add a heater to your Water Garden if you live in a place that gets colder than this to keep your betta healthy and lively!