Geoff -
Here's some info from another post I made a while back:
I found these tires at tires-easydotcom for $75.40 ea., shipping is $10/tire. They offer the same size as the stock GH bar tread tires, so fitment should not be a problem.
These Carlisle All-Trails are the same ones I put on back of my Deere GT235. They have excellent traction and I'm thinking they won't tear up the lawn doing the zero turns as much as the stock bar treads do.
As far as tire-changing at home... with the proper tools it can be done but it depends on how easily those old skins will come off. If they're the original tires then they will likely be a royal bear to get off the rims. The good thing about doing it at home is that they don't need to be balanced. Once switched out, just dump in some Tire-Slime and inflate and they should be good to go. I know the Carlisle All-Trails have very good traction and since I have such positive experience with them on my Deere I'm going to try and pick up some used rims and mount a pair for my 'Hopper, too. Then I can run Carlisle-mounted wheels in the summer for mowing and simply swap back to the bar treads in winter for snow removal.