As mentioned, the souvenir mugs do change fairly often at DLR and not that many are unique to DLR (many of the designs are available in both WDW and DLR, just not necessarily at the same time). I think that pretty much only the Dinoco Sippers and Marvel mugs are unique to DLR.
The Dinoco Gas Can sippers are available at the Cozy Cones in Carsland. They come in three colors (green, blue, and red). They're pretty cute, but they don't have a handle, so they're a bit bulky to carry around. These are what they look like:
Tomorrowland is a great place to find Star Wars and Marvel themed souvenir cups. There is the Chewie mug for the Star Wars fans. They used to have a Boba Fett one (but I don't recall seeing that one in December). They've also had an Iron Man mug in the past (Redd Rockett's Pizza Port).
Not unique to DLR, they were heavily featuring the Olaf sipper mug in December.
Award Wiener in DCA also tends to have some type of featured sipper. It changes fairly often, but it's another good venue to check. The last time I stopped by, they had the Crush Sipper.
I don't know if Bengal BBQ still has the Moana Sipper, but they were featuring those in early-December.
The Golden Horseshoe is another location where they sometimes have souvenir drink mugs that are not featured at any other location (for the longest time, they had the Lone Ranger Boot and then the Dragon Stein, but both are now gone). I think they only had the standard mugs in December, but it's always worth checking out the Golden Horseshoe.
As mentioned, depending on when you go, there may be seasonal mugs (e.g., the Poison Apple mug or Mickey Elf Stein). Those can change from year-to-year (sometimes the same design re-appears, other times they get a new design).
Ultimately, it's worth stopping at every QS location you walk by because you never know what they'll have. Most of the time, it will be the same thing that is pretty readily available, but once in a while you'll find something unique/different.
