The online petstores sometimes have sales. I like Dr. Fosters and Smith, and Big Al's. Your local big box petstores like Petco and Petsmart will have canisters. Look out for holiday sales. There's also something to be said for supporting your local fish store.
It sounds like you are on the right track. We wish you and your turtles all the best. Please keep in touch and post some pictures if you can. You sound like a responsible turtle parent