Amazon has the Porter-Cable 4-1/2" 120 Tooth Plywood Cutting Circular Saw Blade (12057) on sale for $4.35. Shipping is free with Prime or $25+ orders.
CPO Outlet also has the Porter-Cable 4-1/2" 120 Tooth Plywood Cutting Circular Saw Blade (12057) on sale for $4.35. Shipping is free on $49+ orders, or sign up for emails and get free shipping on your first order (not sure of details/restrictions).