Purchase a ribbon flower at a craft store or make one yourself. A flower made from ribbon produces a seamless look with your bow, since both are made of ribbon--use the same ribbon for both to get a completely fluid look. You might find that shaping ribbon into a flower is more challenging than shaping fabric or just using a silk flower, however, if you want a subtle, customized look then the extra effort is worth it.
Silk flowers from a craft store are commonly used for floral arrangements, but they also make lovely additions to hair bows. They're also one of your easiest options for adding a flower to a hair bow. Cut the stem from a silk flower and glue the flower to the center of your bow, using a hot glue gun. Choose a silk flower that's smaller than the bow, so as not to completely hide the bow under it.
Fabric flowers are a fitting choice when you want to add a pattern or texture to your hair bow, since fabric comes in a wide variety of both. You may be able to find fabric flowers at craft stores, but these are more commonly homemade items. There's a wide variety of fabric flower tutorials online and there are also fabric flower patterns for sale in many places where patterns are sold. Fabric flowers offer the largest variety of options for your hair bow decoration, because there are so many different styles of flowers you can make with just as many fabrics to choose from.