Check out the projector or projectors to see what types of reels you need. The two most standard sizes for reels are 16 mm and 35 mm. Some companies may only offer one of the two options, so before you start purchasing make sure you know what you will need.
Contact some film distribution companies. These companies can help you if you'd like to rent or purchase film reels. You can also get permission to show films if you want to do a public viewing of your movie reels and need licensing help.
Locate film reels at antique stores. There may be some hidden gems in vintage shops, antique stores and even flea markets. Hit up any of the ones near you, and look up others online to see if they have what you are looking for. Garage sales and estate sales could also be a good option for finding reels.
Go to the bookstore and load up on film and cinema magazines or publications. Flip to the back to look at advertisements and see if there are any for film reel companies, vendors or private sellers.
Ask people you know if they have any film reels gathering dust in their attic or basement. Family members may have some reels from long ago that they forgot about and would love to watch. Friends may have reels they don't need anymore or don't use that they could give to you.