Just add a few small chunks about once a month and the turtles will nibble on them as needed. The first three foods mentioned above can be obtained at just about any pet store and Turtle Brittle can be ordered from a company in Wisconsin. I learned about Turtle Brittle from the vet in charge of reptiles at the Tennessee Aquarium in Chattanooga Tennessee. This is the largest freshwater aquarium in the United States and has an extensive turtle exhibit. It is also recommended by the Atlanta Zoo and Herpetological Society.
Filtration is very important.
There is not a specific formula on the exact amount of filtration needed and there are so many ideas and discussions on the subject that I could write about them for hours. I will just tell you what I think. You can never have too much filtration in a turtle habitat. I strongly recommend having a filter or filters strong enough to filter at least 3 to 5 times the amount of water to be filtered. I personaly like to go with about 10 times the filtration.