No idea if they're available.
Best bet may be a breaker's, take the one from the other light unit with you, they will be the same (or close enough) across many other makes/models.
Wouldn't be too hard to make one either.
Get a length of Spring Steel Wire (AKA Piano Wire) from a Model/Craft shop, and bend to shape with pliers, copying the one from the other unit.
You'll need 1.4mm diameter wire, that's 0.055" (55 thou) in old money, or 17 guage (SWG).
Piano wire
In the meantime, a bit of foam stuffed under the rubber boot will keep it in place.