I agree that the 4 ply rib option would be a good tire for your application. An aggressive tread may scuff your lawn a bit in sharp turns.
I bought front tires for my 1250 Nightmare from a local tire distributor. They were far cheaper than ordering them from an on-line supplier; and the rib style was very similar to the original Good Year tires that they replaced. They are 4 ply tires.
Here is the link. They may be cheaper.
http://www.fargotire.com/