There are two different type washers used in Flex Boots: a single washer and a double washer.
Single washer is used for the pastern strap under the ball rivet, and double washers are used for the shell of the boot under the ball rivets.
Washers come as part of the standard Flex Boot set when you purchase a boot, or can be purchased separately as replacement parts.
If you need to replace a ball rivet on the boots, check whether your washer is still intact or if you need to replace that too. It is not recommended for rivets to be attached to the boots without the washers, and this may damage the boot and will void any warranty.
Single washers are sold in a pack of 2, and double washers are sold in a pack of 4.