My red eared slider baby won't eat?

I've had my baby turtle for about two months and for some reason he'll only eat the food that his original owners provided for him which are these little round brown pellets that I can't seem to find anywhere. I'm worried about his diet so I've been trying to feed him other things that turtles like such as carrots, berries, dried shrimp treat, reptomin food sticks, and romain lettuce, but he refuses to eat it. I'm really worried that his health is deteriorating because he'll only eat the pellets which I'm running out of. Any advice?