I assume you're talking about the c-clips on the three pins of the diamond linkage, and not the larger circlip around the cable clamp mechanism.
In either case it shouldn't be a problem getting hold of one. Try a model shop, one that specializes in remote control cars and planes. If you're in the UK, try Yell.com (online yellow pages) to find such shops nationwide.
Custom Fasteners in Newport, Wales supply circlips and fasteners for classic motorcycles. How small they stock I don't know.
Failing that .. just find a mild steel washer of identical thickness and internal diameter .. pop it in a fine jawed vice and file off ~1/3 of it so it becomes 'open' and resembles a crescent moon. If you can hold it sufficiently steady, you could achieve the same more quickly with a small hacksaw.
That would do it.