Grease the new ones regularly, when you get them. Helps keep them in service longer. Also if your carrying weight on the front, grease them even more.
I just go to my local small engine shop and he has them in stock. They fit numerous different tractors and other wheeled things.
I had to replace the wheel bearings on my MF 14, I got mine at Tractor Supply. Mine came out in pieces. I found them in the hardware section, in the bushings/bearing drawer. They seem light duty to me for a garden tractor but they do the trick. Good luck!