I have a couple Husqvarna or Ariens 42" mulching kits for riding lawn tractors. The model number is HU42 on it, or possibly 531-30-96-41.
These are new in the box and come with the blades, mulching plug and hardware for the 42" mower deck. These are $50-55 in store. I believe they fit 2007+ tractors according to this: http://www.mowersdirect.com/Husqvarna-531-30-96-41/p4576.html
$30 each picked up in Rochester. Can ship. Cheaper than buying replacement blades alone.