Try eBay. I has the same problem with my GPA, buying a new liner helped.
You don't have to sell your GPA obligatory ! Also you could buy something to put inside to fill the blank space !
could try tackhunter.. its an app.:)
English Tack Trader I've noticed the GPA speed airs sell really quickly on there.
Craigslist or there are a lot of buy and sell Facebook groups that are for horse/equestrian supplies.