After you battle in the arena, you will have access to North Bowerstone where Lady Grey lives. In the middle of the area, marked in gold, is a shop with a guy that sells rare and powerful weapons like the Master Greatsword and master Longbow for at least 18,000 gold cheaper than that guy at the Hero's Guild. Be careful what you buy, after you do business with him one time his shop closes and goes up for sale at about 150,000 gold.