If you subscribe to your local newspaper, check with your delivery person to see if he/she has any extra coupon inserts. Many times the delivery person will have extras, and be glad to not have to throw them away.
Make sure you don't clip off the expiration date on your manufacturer's coupon. Many stores who do not accept expired coupons will not accept a coupon unless the expiration date is in sight. Please note that on some manufacturer's coupons, the expiration date is also noted in the same section of the coupon that tells the store where to send the coupon to.