Just in case you don't know what nls is its called "new life spectrum" get the betta formula. It's honestly the best food I have found, by it is a bit stinky, and pricey, but lasts forever.
Omega one is also a really good option. It comes in both pellets and flakes.
Pellets are much better than flakes, IMO and 99% of bettas are used to pellets so they don't pay any mind to flakes.
My sorority is in a community tank, and so I feed my sorority NLS, and the community some omega one tropical flakes, my bettas always spit out the flakes, I'm guessing they just don't like the taste on them?