I've gotten some really cute ones at
Walmart; they have some Mickey & Minnie Valentine's Day pjs right now, too.
If you have a local Kohl's, I've found some of my favorite Disney shirts there, too.
I've had some awesome finds. Two of my favs are an Alice shirt (old, cartoon Alice - not new Alice) from Kohl's and a cute one with all seven dwarves on it from Walmart. I wore the one with the seven dwarves on my last trip, and got stopped all day long in the park by people asking me where I got it.
They're always very affordable at both places, too!
I know Old Navy had some child tees before Christmas, but I'm not sure how often they get those in. I've even seen a few things at JCPenny. I also know Hot Topic had a lot of shirts from the newer Alice movie when it came out, but I'm not sure if they still carry them.
Really, Disney is everywhere anymore!
ETA - I got my mom a Disney snuggie at Walmart right before Christmas. And, funny enough, my mom got me a Mickey one from Kohl's. The one from Kohl's is insanely cute; it's made to look like Mickey is hugging you.