Visit a carpeting store. View samples and place your order for your specific square yardage.
Contact the manufacturer directly. If you need to purchase a large quantity of carpeting in different square yardages, you may want to contact the company that made the carpeting you're trying to purchase. You can find this information after viewing carpet samples at a carpeting store or by referring to carpeting manufacturers' websites.
Buy your carpeting from a home improvement store such as Lowe's or Home Depot. These hardware stores sell carpeting but they may not have the same type of selection that a dedicated carpeting store has. They will, however, have the ability to order carpeting in different square sizes to match the physical dimensions of the room you're remodeling.