Call for Artists, Making Money at Juried Art Fairs, Craft Shows and Festivals
Have you checked Walmart to see what they have. They probably have the best selection available in stores.
Hobby Lobby has a storage section and I have bought from them many times. Usually a 40% off sale rotates or you can use a coupon.
While at art fairs, go and visit the other clay artists at the fair and ask them what they do and use. They are your finest resource.
Emily Brown ... I once had a canvas bag on wheels that held plastic bead storage boxes. It came from JoAnn Fabrics. The plastic storage boxes can usually be reconfigured to hold the size you need.