Yep that sucks with cowboy tyre fitters ...
I normally try & take the caps off myself first before taking wheels into a tyre shop. It's also handy to have a spare set of wheels to swap out, so you can then just drop off the ones that needs fixing & still use your car ...
Haven't had any luck yet with finding any more of the center caps yet, will keep searching eBay etc. for them & will let you know if I come across any ...
P.S. - Apologies for the images not working by the look of things - I will have go back & fix them later ...
Cheers, Bantum ...