I lost mine years ago.
I took a small piece of 14gauge copper wire (household wiring with the insulation stripped off of it) and bent it around the groove in the end of the swing valve stud with needle nose pliers. It held up for many years and was re-usable. (It was easier to find once dropped on the garage floor also!)
Might be worth checking you local hardware store. ACE Westlake and Bomgarrs here locally carry these in their bulk fastener area. Usually there is an isle that has random stuff like this.
(everyone has their favorite shop. you get the idea...)
in the meantime, hardware stores have e-clips which work fine - home depot, lowes, menards, ace, ect. Find them in the pull out drawers, probably a couple in a bag for less than a buck. a couple years ago I grabbed some for a member to have as extra... two for $0.78.