Not many Bettas will take flake food, as the above poster mentioned, Hikari bio-gold, it's the best dry food out there, they are pellets and most Bettas will take them. Then if you can, frozen foods such as bloodworms and brine shrimp.
I also feed hikari betta bio gold.. I also give them flakes, cichlid granules, frozen bloodworms and peas. (If you do peas make sure and take off the casings!!! I use frozen and I just soak them in hot water, take off casings and drop it in... they really like em)