I've been looking for these for my '89 Bigfoot
with no luck so far. I have both of mine, and recently rebconditioned them. But they are pretty corroded and rough looking and they are difficult to operate. I wouldn't mind replacing them with new ones.
I haven't found any but I did find this gravel guard support arm that has a similar appearing spring pin riveted on the end of the arm. Without actually buying it and trying it, I don't know if they it would work on the BF but it might be worth a try if you are actually missing one. If you had it in-hand, you could probably measure the spring clip assembly enough to determine if it will be a good fit.
If not, you could return them. If so, you'd have to drill the rivet out to remove the aluminum spring pin but that wouldn't be too difficult.
Just a thought.
Stone Guard Support Arm