First of all, always check out the particular store's website that you are planning to shop at. More and more stores are starting to post their own printable coupons (particularly stores like Target and Walmart).
Also, check the manufacturer's site as well as any facebook pages for possible coupons.
Otherwise, for free printable coupons, here are some pretty reliable sites:
Coupons.com
SmartSource
RedPlum
Coupon Network
