Clearance: Check out your craft store's clearance area. You can find treasures within. Many stores will reduce the prices on items that aren't selling well and this could be the best time to pick up some great deals. Never forget coupons, either. And, also look in the little girl's section of your local stores for reduced-price straight hair ribbons. You may just find what you need.
Think Smaller: Some crafts stores will sell strips of ribbon that they measure and cut for you. You don't have to buy a whole roll, especially if a roll is like $15. Just buy what you will need, plus a few extra inches to give you some 'play' when you are making your own hair bows. You can also shop for remnants. Those same rolls that they cut pieces from often have strips remaining and they usually will sell the strips.
Re-Use: You may not have to even leave the house to find great ribbon to use. Look at older clothing, toys, gifts, etc. that may have ribbon on it. Even some cloth can be re-used to make ribbon strips. You just want the material to be sturdy.
Online: You can check websites like eBay or Amazon for supplies. Many people on those sites are crafters that will sell their overstock at a fraction of store prices. Or, they are using these websites as their 'store-front' and drop ship products to their customers. Don't forget to go to Google and search for stores that sell craft supplies. Just be creative with your search results and look for places that have great testimonials, good prices and fast, inexpensive shipping.
Freebies: If money is real tight, you can try a Yahoo group called . If there is a group in your area, join. After you join, you can search for postings where people may have offered ribbons, glue sticks, etc. These items are FREE. If there isn't anything available, you can post that you want certain items. Please be sure to follow the guidelines of the group. It is a great resource to get your hair bow materials for free.