Clutch master cylinder - circlip won't stay put

I have a 1968 Spider, with standing pedals, and the circlip in the end of my clutch master cylinder keeps popping out. It doesn't seem to have much spring in it so I guess it has fatigued over time. The clutch is otherwise OK but I can't find an online supplier who stocks just the circlip. The master cylinder repair kit doesn't seem to include it, as it is just the rubber parts. Does anyone know of a supplier?