Local tack / feed shops which sell them should have replacements. My local shop had about 10 replacement tops laying around when I last went in at £2 each
I'm sure I've got some somewhere, still in their bags. I'm never going to use them (I seem to have thirteen snack balls and only one horse who will play with one!), so am happy to send them to you if you'd like them?
PM me if you would.