Welcome to the forum!
Being in "O Canada" and getting supplies across the border from the states may be costly.
You can try contacting Agri-Drain and seeing if they will custom make boards for you.
https://www.agridrain.com/about-us/contact-us Or you could see about finding a local sawmill and having them cut some from local native hardwood.
To repair it, could you put boards in the last slot (if I'm understanding your post correctly) let the water level stabilize between that slot and the pond, then remove the boards in the first slot? Yes, they will have water on both sides of them and it will be a wet, cold job. Then once you have those boards replaced, remove the boards on the outer slot, going back to using the first slot as the primary "dam"?
