Welcome to TBN. Been a long time since I thought of Niagara. What do you wish to do with the blades? You will find on the service decal on the tractor part numbers for what MTD has available. OEMs are good blades. You may be able to drop the numbers into a Google search and match to some Oregon blades which may have a selection of both hybrid blades or comparable to OEM. Or post the chassis number from below the seat or the blade numbers so we don't steer you wrong.
I had to sharpen the blades anyway, so I found the markings on the blades, they measure 18 5/8 long and have the number - 742-04416 stamped on them. I found a set on ebay for 45.00 and bought them.