tilt the deck over and take a picture of the bottom side plz?
if it is a single long blade (goes from one side to other side of deck), then entire blade needs to be replaced.
if blade is a 2 blade thing. one blade on each side. for bottom or top set of blades. then would most likely suggest getting both new. either for top or bottom which ever one has the broken blade. it is more about trying to reduce un even weight, and overall force a new blade vs old blade will create, and cause a large amount of vibration. and trying to keep things evenly disturbed across the spindle/shaft that comes down to the blades. with that noted, find bolt/s pattern/s for the blades, or take one off, and take it to a local dealer, they should be able to match something up for you. for both bolt pattern and overall length.