For the lowest prices, compare "big box" retail pricing with specialized discount roofing centers.
Builders Liquidators has been noted for selling shingles at roughly 40% off typical retail rates. Where to Buy Cheap Roofing "Paper" (Underlayment) where to buy shingles cheap
Stores like The Home Depot and Lowe's often offer bulk pricing discounts if you buy 30–50+ bundles. For the lowest prices, compare "big box" retail