Your best bet might be to find a bike co-op near you. Those round reflectors look like Cat-Eye's.

Nice Ross early mountain bike, by the way. It looks new. Gold rims and hubs from the BMX fashion at the time.

Oh, wait, are you looking for the wheel reflector? You can try bike shops in addition to the co-ops. The key is to have your rear reflector to match up the one that you are seeking if you want a perfect match. Those are some big wheel reflectors. At that time they all seemed to be be big. It was probably a mandated rule about reflector area.

If you didn't already, you should post this in the Vintage Mountain bike thread. Show your vintage mountain bikes!