There are two blade kits available that work on it. Kit includes (3) blades & (3) lock nuts for the 54" deck. GY20684 or GY20679 for side discharge and GY20686 if you have the mulch kit. I run those number through my local dealer but they did not list prices for them.
Here's the maintenance sheet for it. They are handy to print out and keep with your other paperwork and receipts.