What do you think of this as a source of live food for your bettas? Ignore the cartoons. They're not that cute, they're just brine shrimp meaning once you get past the age of 10 the excitement of having aquatic monkeys wears off and they become fish food.
Research this well first. Unfortunately I never had any luck keeping sea-monkeys alive for very long even though I followed the manual. And if the sea-monkeys aren't kept healthy they can't be very healthy for your fish. Although I will admit at a time when I was more uninformed years ago and I was also misinformed by pet shop employees I had bettas in glass bowls, no heater, no filter, please forgive me, I didn't know any better back then, I did cultivate sea-monkeys and feed them to my bettas and they were tasty morsels to them. But now I would be more cautious about this since my sea-monkeys did not last long. Try sucessfully raising healthy sea-monkeys first, if you are good at this and they don't die before they're supposed to then you should be able to feed them to your betta.
Oh, and I agree with Sakura, supplement with prepared food.