Buy a 10 gallons tank starter kit. Trust me, i go to petsmart everyday and i dont think anything else is cheaper. But they might be closed on new year. Otherwise, i bought the starter kit LED for about $47 and has absolutely everything you need except for food. And if ther are on sale, it would even be cheaper. It came with heater, filter, net, lights, and a cheap thermometer (but it should work for now). Too bad you miss their Christmas sell which is the exact same thing for $30. Buy a few silk plants, no need for gravel now if you don't have the resource.
Please don't buy anything less than 5 gallons for both of them. Even a 5 gallons would be fine for a short run, but Petsmart 5 gallons cost $30 without a heater which could potentially cost up to $20 so might as well buy a 10 gallons right?
And for food, for now, the betta pellets might be sufficient. In the future, you should buy variety of food like frozen/freeze dried brine shrimp, bloodworm, daphnia etc.