arty Farnum can't keep pellet stoves in his Northern Lights Hearth Sports store on Wilton Road in Farmington. Nor can Mack Curtis, who owns Wayne's Stove & Canoe Shop in Madison. Ditto for The Stove Barn on China Road in Winslow, owned by Wade Bullard. Heating-oil prices that could flirt with $5 a gallon this winter have more people ready to switch away from oil. For an investment of roughly $3,000 plus the cost of pellets, a pellet stove might pay for itself in a year or two, shop owners say. Mainers appear to be believers, and they're scooping up...