Pretty sure @Slestak75 knows a lot about shims based on the mods they’ve been doing good luck. If you measure the other shim, that might get you closer to finding a replacement also. Also try to email c3 maybe and ask
Can you remove the other shim and get pics? Is the shim flat? I’ve been using some shims that fit a standard C bearing post and these might be suitable replacements. Would depend on how thick it needs to be as well as the size of the inner hole diameter. Also GT makes a good point. You may contact G3 as they may have exactly what you need.
It’s not a shim, but a specifically shaped spacer with a step on it. I thought I’d damaged one in my Gamma Crash and asked around but no one had replacements and C3 never responded directly. If I were you I’d get a set of kneepads and a flashlight and start hunting.
This is what I was figuring would be the case. It is a possibility that you could take the one you have to a local machine shop and have them make a spare or two. I don’t imagine that would be crazy expensive but definitely could be more than one would be willing to spend. In that case it would really come down to how bad you want it. But it wouldn’t hurt to call a local shop, or stop by and ask. They may be happy to get a few dollars for a piece of scrap. Definitely do all you can to find the one that’s lost. Sorry I can’t be of more help.
When I lost a spacer for my Top Yo Cloud, I took a chance that the Thesis DaCapo shims that are sold separately might fit. They did.
It makes sense, since the dimensions of the bearing post and the outer diameter of the C bearing are the same. The potential difference is the thickness of the spacer, but this was close enough for the Cloud and DaCapo
Hopefully the picture shows that the Cloud spacer is clearly stepped, while the DaCapo spacer tapers towards the outer edge to achieve the same effect.
An obvious hazard to experimenting here is if an off-spec spacer gets wedged either on the bearing post or the outer edge. For both of these yoyos, the spacer just drops in and out.