Finding free fairy costume patterns online can be done, although if you have something very specific in mind, you may need to explore other options. One convenient aspect of fairy costumes is that the look can be whatever you wish, so having a sense of creativity and invention can be even more important than the ability to sew.
Basic Free Fairy Costume Patterns
A lot of the free fairy costume patterns are less actual sewing patterns than instructions for putting together a simple costume for a child. A good example is the Tinkerbell costume which is available on SewDelish. This is a simple costume the crafter made for her daughter and includes dress, wings and slippers. She notes that demand is so great, she is now in the process of creating a separate, detailed pattern for the fairy wings, which should be available very soon. All the dress components - bodice and skirt - are cut in rectangles and then fitted. Her instructions are clear, but she is available for questions should you need help.
Make Your Own Pattern
One guaranteed way to get a free fairy costume pattern is to make one yourself. That may sound intimidating to all but the most experienced sewers, but for a simple costume, it is actually fairly straightforward. The instructions at Get Cool Costumes suggest using cardboard to draw patterns of flower petals, leaves and wings for your basis. If your daughter is already in a ballet or jazz class, so much the better. You can have her wear her leotard and tights and possibly even a tutu, or just use filmy fabric to construct a skirt. You can even just tie multi-colored ribbons to a piece of elastic and stitch it at the back around your daughter's waist. Using your cardboard patterns of the flowers, you can cut them from multi-colored fabric and baste them to the leotard and skirt.
You will need bigger pieces of cardboard for the wings. Wings can be in any multitude of sizes - get your daughter to help you decide what size and shape she would like. Take your time drawing them, as they are arguably the most important part of the costume.
If you want a much higher quality pattern, similar to those made by Simplicity or McCalls, or even something more elaborate such as a Cosplay-style pattern, you are much less likely to find them available as a free download online. Instead, you will have to be more proactive about securing the pattern of your choice. If you are an active sewer, you are probably a member of a sewing club or circle of some sort and that is a great place to ask. Or, if you're part of an online forum, you probably need only post the request and will receive several patterns within a few days.
You can also post requests on such online sources as Freecycle, which is dedicated to the world of easy giveaways that keeps things out of landfills. Even Craig's List has the capacity to post requests for free items and you might be surprised at what people have that they are willing to give away or swap for something in kind.
Finally, you might try contacting your local community theatre group or college. They are always looking for free things themselves, but they will usually have a stash of basic patterns on hand. If you have old costumes or props to swap, they will be happy to work with you. Or, of course, they can just lend you the pattern if it's not an original design and you can return it when it's completed.
Finding a pattern for free can take a bit more effort than just going out and buying something, but for a good homemade costume, it's worth it to save the money for better fabric and accessories!