I have no idea. The tractor came with a Sears manual 3PH so it is possible the wheel weights are Sears also, but truthfully I haven't researched it.
The next time I pull the tires off I'll see if there is a part number on the wheel weights.
Btw, I just searched the Ebay for "MTD 990 wheel weights" and got a few hits. There are several folks on Ebay who sell 990 parts so definitely consider Ebay a source for parts new and used.