You can get the small caps from Dickinson Marine -800-659-9768. They make a stove that uses the same small cap, p/n 89008.
You are out of luck on the large caps. Force 10 sells a kit to replace the entire burner assembly with a stainless steel burner for around $125. Right now that's the only replacement option.
I repaired my large cap, which had a chunk missing from one side, with heat resistant ceramic paste. It comes in a tube and adheres pretty well to the cast iron. After it hardens, you shape it with a file. I don't know how long the repair will last, but so far, so good.