Join a free recycling group. Recycling groups are a wonderful way to find anything you may need. All you have to do is make your request known to the group. You'll never be charged for these items. In fact, people who join these groups are looking to get rid of items, without throwing them away.
Sign up at websites. Your favorite food manufacturers may have free cookbooks available. All you have to do is visit their website and sign up. While these aren't glorified editions, they do contain useful recipes for products you already buy.
Take advantage of promotions. Book clubs often have promotions that allow you to get cookbooks for free. You usually buy one and then get several more for free. This is a great deal if you love to add to your collection on a continual basis.
Post an ad. Some people are willing to give away their books for free. These people may be moving or may just need extra space. Usually, they don't want to bother with selling their collection, but throwing them away is out of the question. In other words, they're happy to give away their collection to someone that will use it.
Ask friends and family. It never hurts to ask your friends and family if they have any cookbooks to give away. You may find that your mother or grandmother has memorized her favorite recipes and no longer needs her collection. Others may have been given books that they've never used. In either case, you're bound to find a freebie or two.