The best recommended heaters . And you can get them for one gall i think it 25w.
Penn Plax Cascade
Hagen Elite Mini Submersible
Most of, if not all of these heaters have to be ordered online or found in a local fish shop. Petsmart and Petco generally don't carry them. Good places to order are www.amazon.com
I don't think you can find that betta food at walmart. You need to go to Pet stores. And the stores around me sometimes out of stock so i have to wait for new delivery. And not all stores have those brands. I know my petsmart have Omega but don't have NLS. So check a few stores , but i guess if you can't find you can order on line. So don't even spend you time to go to walmart.
When you give him flakes just try to see if he eats it. I know you said that he is hiding when he sees you. But if he is healthy hopefully soo he will get used to you and will follow your finger too. I guess just get away from the tank close enough to see him. If he is healthy and eating he will come to eat.
Give him a little pinch of the flakes. Just little bit because most of it sink to the water right away.
Also when you go to pet store get aquarium salt and if you will be at the pharmacy department get Epsom salt (unscented) . It first aid for bettas when they sick . I think it good idea to have them on hands