First and foremost, there's your local newspaper. Sunday's ads are usually the best, packed full of different coupon inserts. Most local newspaper systems offer a subscription for just the weekend, if not a Sunday only option, to help you ensure you get all the coupons. Don't forget magazines you regularly read as well. You may be able to find out what coupons are in what magazine by doing a little research online so you don't spend money buying a bunch of magazines.
After the local paper, try online. There are many different coupon sites that offer free printable coupons for your local area, based on zip code. You can choose to clip and print individual coupons, or an entire batch.
Your local grocery store. As you walk through the store, you may find coupons presented on items or in a dispenser near an item. Though they may or may not help you with the current shopping trip, they are good to have on hand for the next one. Don't forget to check the sales paper for store specific coupon deals, too.
Watch the commercials for specials run by companies promoting a new product. They may tell you to go online, to a specific website to print your coupon. You may have to spend a little extra in printer ink to take advantage of these deals, but many of them are worth it in the long run.
Products you buy. Sometimes products will have coupons for another purchase, or the purchase of a related product. Keep them. Use them.