Spells can be cast for free by writing a spell and then working the spell using the previously decided upon materials and method. Or, consult with members of a religion that use spells to get advice on what spell may work for the intended purpose.
Most followers of religions that use spells do not believe spells need to be purchased. In these religions, such as Wicca, any adherent may write their own spell for whatever purpose they choose and include whatever materials they feel are appropriate for their intended purpose. Many Wiccans use items they have around the home for spell casting purposes, including candles, figurines, plants and spices, and write their own incantations to say during their ritual.