Taxidermy.Net is a pretty good site. They have a section for people looking to sell items and to buy items. You can register for free and do a post on a polar bear. Not sure what you'll find though. Full mount polar bears aren't the most common item. Even when they were hunted without restrictions, not many people got to take one due to where they live. I'm willing to bet that 90% of all the polar bears ever taken were made into rugs and not full mounts. Not many people have the room for things like that.