Amazon offers the Arm & Hammer Easy-Tie Pet Waste Bag 75-Pack for $3.99. (Blain's Farm & Fleet charges the same via pickup.) To score free shipping, add it to your cart via Subscribe & Save. That's at least $1 less than you'd spend at a local retail store. These bags use baking soda to neutralize odors.

Note: Remember to cancel your subscription once your order arrives to avoid being billed for subsequent shipments.