Humphrey is the mascot of the Wilderness Lodge, so he's on a lot of the official merchandise (see some here).
To double check if they sell the plushes, you can contact Wilderness Lodge directly at 407-824-3200. Just explain that you're wondering if the Mercantile sells a certain item, and could the switchboard please send you to the right cast member for that information.
Or you could contact Walt Disney World mail order at 407-363-6200 (Email: [email protected]). Again, explain that you're looking for large Humphrey the Bear plush stuffed animals at the Wilderness Lodge Mercantile, that you don't have the sku number, but you're wondering if they can help you out.
Good luck!
WildernessLodgeSite.com
An unofficial tribute to Disney's Wilderness Lodge and Villas