Are you talking about the faux wood looking type mugs-- I have seen a Country Bear mug like that, or just in general? In general, the answer is pretty much anywhere. The resort gift shops often have a bunch of mugs (at one point DD wanted about 5 in the Yacht Club gift shop) and any of your big parks stores (Emporium etc) usually carry a bunch of mugs. If you go to Disney Springs as the PP said- world of disney.
If you are talking about the ones that look like wood and want unique things or shapes--- Trader Sams (poly) and Oga's (Galaxy's edge) usually have those type. Trader Sams had a Monstro the Whale one recently that was really cute. Know that those type are pretty heavy.