It seems the big box stores (Lowes, Home depot, Menards, etc.) have replacement blade kits that may work. Then there is always the Trekwood (expensive) option. Here are a couple of Threads from the past about broken fan blades.
https://www.montanaowners.com/forums...ad.php?t=71535
https://www.montanaowners.com/forums...ad.php?t=54711