I used to buy them in Freedom, NH at LandPower. They have since gone under (so I have heard). Last summer, I bought a new air filter, oil filter and fuel filter in Sabattus at Waterman Farm Machinery. I know the fuel and oil filters are relatively inexpensive, but get ready to shell out about $60 for a new air filter. My father used to do business at Abbot's. I say "used too" because they are twice as expensive as anyone else and he found them to be really rude. When I was shopping for my first tarctor (Cub 7200), Hazelton's in Cumberland wanted $13000 and something, LandPower wanted $12000 and something, and Abbot's wanted $15000 and something. It didn't take me long to decide who had the biggest markup. If you don't mind a ride, go to Waterman's. Just get on Sabattus Street in Lewiston and keep going. They are on the right side. They have a dairy farm across the street. You can't miss it.
Good luck.