You have a few options for making fake ones, if you can’t find the size you need:
– Find plastic globes and cut out the slit shapes. A hot knife used carefully in a well-ventilated area would be a good tool to use for that. You can then paint them the color you need.
– Seal and paint styrofoam spheres. I would look for the smoothfoam rather than the bumpy kind with all the holes, and use spackle (sanded down after each coat) for smoothness.
– If you can find hemispheres (of foam, of plastic, etc.) or make them, you can attach them together at the center to do the same. These will be more easy to store if you need very large bells, since if you make the center attachment detachable, you can take them apart and store them stacked inside each other.
– If you need something very lightweight and large, you may be able to use vinyl inflatable balls, though these will be much harder (or nearly impossible) to paint if you need a metallic color
– If you need a very large size, you can use a fabric in the color you need and make plush bells. Look for ball patterns to get the proper roundness. If you need metallic-looking bells, you can use a metallic spandex or vinyl, and then applique on the slits.
Finally, if the bells are a somewhat reasonable size, look for bells sold for fans of the book Polar Express. You can get them up to a few inches in diameter, and then if you need to disallow the ringing, fill them with a bit of hot glue. I’ve used these on a costume before, though I didn’t need anything hugely oversized (not like Deijiko from DigiCharat or anything haha).
I hope that helps! Good luck :]