If you have the lens, I'd just measure it and find on that auction site. That's what I've always done. Get yourself some cheap calipers from Harbor Freight or online. They're one of those tools that you don't think you need until you own one, and then you wonder how you ever lived without.
If it happens to be a weird size that you can't find (or can't find an affordable option) you can make one out of Worbla or some other thermo plastic. I recommend the black variety. Just cut it larger than you need, heat it up, and push it over the lens to form the cap. If it's too big to stay put, either line the inside of the lip with felt, or heat it up again and push work it in further by hand. If it's too thin to hold it's shape, then you can just heat up two pieces and smush them into each other to form one, thicker piece. That stuff is really useful for weird projects like that, though it's not cheap. If you have a friend who's into cosplay, they may have some scraps they can sell/give you. Or put an add out to exchange some cosplay photos for some Worbla scraps. Cosplayers are always looking for cheap ways to get good photographs of their costumes.