Find the best prices for shingles per square by contacting a local contractor and asking to purchase his overruns. Another option is to contact a roofing manufacturer and ask about any overruns, cosmetic seconds or past-season shingles.
One option is to purchase shingles on clearance at home improvement stores. Ask a floor manager for options the store is eager to discard, including end-of-season closeouts, discontinued models, and unpopular colors or styles. If the store does not have a current stock of clearance materials, ask the manager to contact you during the next sale.
Another option is to consider purchasing materials on an auction website. It is harder to find good supplies this way, but overrun materials are sometimes available at a good price. Check these sites frequently, because new listings are uploaded every day. Check the seller's reviews, and do as much research as possible before committing to a purchase. Ensure the seller is a reputable person or business to avoid making a bad purchase.
To determine how many shingles you need to purchase, calculate the total square footage of the roof. Take that number, and divide it by 100 to get the shingle amount in squares. Add 10 percent of the total to have a trim allowance, and purchase the same amount of material for underlayment.