Craft and hobby shops, some hardware or home improvement centers and many on line sites!
www.klockit.com is a good supplier
Michael's, Hobby Lobby, www.rockler.com.
Michaels, JoAnns, Hobby Lobby, At Home all sell clock parts
Hobby Lobby, Michaels, Amazon
You can purchase those kits online and generally in many lengths for different size clocks.
Dan .... I have seen them at Hobby Lobby.
Hit up a goodwill or garage sale and remove one from an old clock you can probably get on the cheap