There are two types of popcorn buckets: the premium souvenir bucket and the souvenir bucket. The regular souvenir bucket are cylindrical containers. I've found that many of the stands have the same one, but if you look around you can usually find a couple of different designs. So it pays to look at every popcorn stand you pass.
The premium buckets are usually character shaped buckets, like the Mickey Ghost shown earlier in this thread. They typically have a premium bucket for Halloween and Christmas. Last year, the Mickey Hatbox Ghost bucket premiered, and my guess is that will reappear this year. There was also a Santa Mickey bucket at Christmas time. I've also seen R2D2 (DHS maybe or Tomorrowland), McQueen, Darth Vader (debut for Star Wars weekends, not sure if it's around otherwise), and Dumbo premium buckets. If you were going to
Disneyland, I could tell you where they were sold, but I don't know where they are in WDW anymore.
This is the R2D2:
This is the McQueen (ignore the blue one, that was at DLR for the holidays). From what I can tell, they might only sell McQueen as part of the kids meal at a few places in DHS. It's sold as a popcorn bucket in DLR, so I still consider it a popcorn bucket!
This is Darth Vader:
This is Dumbo:
This was the Hatbox Ghost bucket that debut last year for Halloween: