Try New Life Spectrum betta pellets, or Omega One Betta Buffet as they are the top pellets out there (more ideal then flakes for a few reasons), and they are generally loved by picky eaters.
Bettas can take weeks to adjust and eat healthy once you bring them home.. just continue to offer the food twice a day, and remove any uneaten right away to avoid fouling up the water and causing ammonia spikes.
Freeze dried lacks a lot of essential nutrition and vitamins, and should not be fed as their main diet.. only as a treat once or twice a week.
If your tank is a gallon or larger then you can safely heat it for as cheap as $8.. anything under a gallon you cannot safely put a heater in it, as the risk of overheating is too great. Small tanks fluctuate too easily to heat properly.
Originally Posted by JBosley
You 100% need a heater unless your house or room the fish is in is always at 76F.
Tanks will be a few degrees cooler then the room temp, and that is being generous. A room kept at 78 will result in say a 2 gallon being between 69-72... too low for a healthy fish. In order to keep the water at the right temp using just room temp the room would have to be closer to the 90s.