I just bought a box these last week while picking up some dog food at a local pet store:http://www.thehonestkitchen.com/products/beams/
" onclick="window.open(this.href);return false;
They are pretty tough and meant to be something for dogs to chew on (like a pig's ear) but can be cut up into small pieces. Not sure if it will work for the cat unless you can smash it into smaller pieces? In the past, I looked for and bought small packages of cat treats that are just dehydrated fish (from Canada) - no other ingredients. I used the dried fish for box work (hiding treats in a box for my dog to find) since they are pretty smelly and I only need to use a tiny amount.
Another thing you can do is to put a piece of this dried fish in a container with some dry biscuits (like the Old Mother Hubbard ones mentioned by the others, or with Charlie Bears) and let the fish "stink up" the biscuits so they are more appealing
I have also used mozzarella cheese (made with skim milk) – the block kind, NOT the string cheese. I dice up the cheese into tiny cubes (less than 1/4 inch) and sprinkle a tiny bit of flour to coat them so they don't stick together.
Kibble is always good to use too - I usually have a mixture of things in the treat bag and like to use the cheese and extra stinky treats (like the fish) as high value rewards.