The current Hard Shell Armour and Wireless Charging Enclosure come with three o-rings – two that are needed, and a third to provide redundancy. Sometimes people want to get extras in case one breaks.
The size is 26x1mm, and you can find them at local hardware stores, or buy more here. Note that they come with silicone o-ring grease on them, which add both lubricity and additional water resistance.
Note that they will not work on the older Hard Shell cases that use the fatter o-ring that holds the top and bottom together.