Those are known as snap rings or circlips. They come in two basic types, internal & external. If the snap ring goes in a groove on the outside of a shaft, then it is an external snap ring.
Both Internal & External snap rings come in many different styles. Most are installed & removed using "snap ring pliers" which are basically pliers that have small round shafts at the end. The pliers also come in internal & external type. The particular snap ring in the picture is a type that should not need the special pliers.
Snap Rings are available from industrial suppliers, power transmission houses (guys that sell bearings, gears & belts) and good hardware stores. My local Ace Hardware has them. Grainger has them. Mcmaster Carr has them. MSC has them. The hardware store will usually sell you just one. The industrial supplers may make you buy a pack of 10, 50 or 100. They do come in stainless as well as carbon steel.
When you specify the diameter of a snap ring, I believe that you measure the minor diameter of the groove that it will ride in. They come in different thicknesses as well as different diameters & styles.
If your shaft was 20mm diameter & the groove 19mm, then this might be a good choice -https://
www.grainger.com/product/5CY72?cm_mmc=PPC:+Google+PLA&s_kwcid=AL!2966!3!166592851584!!!g!82128319557!&ef_id=WhCuzgAAAFY4-yVM:20180818022551:ss_kwcid=AL!2966!3!166592851584!!!g!82128319557!&ef_id=WhCuzgAAAFY4-yVM:20180818022551:s
I don't know the actual diameters you are working with.