Buy them from someone who makes sails for the winners in the class. Similar issues of high speeds and small apparent wind angles.
Should mention that ice boats don't point worth beans - tacking angles to the true wind are about 55-60 degrees. But boat speed is such that the VMG made good toward the weather mark is greater than the wind speed. Ice boats also tack down wind. My son in his first ice boat race did OK getting to the weather mark, then headed for the leeward mark and let out the sail to run downwind. He was lapped by the other boats before he got there. After the race he said, 'Well, that was a learning experience..'.