I second the recommendation for Omega One Betta Buffet Pellets or New Life Spectrum.
I think I know why you did not see those at Petco. They are not kept in the betta area (at least, not at any of the stores I have been to, and I have been to MANY!). You need to look over in the area where all the aquariums with the live fish are kept. There's an aisle near there that has many kinds of fish food. Both Omega One and NLS (make sure it's the betta formulas, both companies make other kinds of food) are usually on the top shelf, IIRC. You may need to ask an employee. Incidentally, that's also close to the area where you will find the better water conditioners like Prime and Stress Coat, as well as aquarium salt (usually they are on the next aisle).
The sad truth is that most other betta foods are garbage. Take a look at the packages of what you have. What's the first ingredient? If it is not a kind of fish or krill, or somesuch, it's not very good for them. Bettas are carnivores/insectovores in their natural habitat. Here's a tip to help you with costs: make sure to sign up for the Petco Pals rewards club. It's free, and they will send you coupons in your mail and email for like 10 or 20 percent off.
I am very happy you found this forum and good luck with your new finned friend!