Your best bet would to go to your local Build a Bear store and look for stuff there as most of their outfits and costumes fit bears up to 20" I think.
But the "My First Duffy" bear *is* pre-dressed and with most of the pre-dressed 12" bears, the costumes are NOT removeable. The sleeping cap he wears in the "My First Duffy" version is sewn to his head and his jammies are probably attached in some way too. The only 12" pre-dressed bears I know of where you CAN take off his clothes are the WDW 40th Anniversary and the New Years ones as they just wear hoodies. I just got myself a WDW 40th Duffy to add to my collection.
If you're careful, you can "snip" the clothes off of the pre-dressed Duffy, but you have to be really careful. There was one released at Tokyo DisneySea that was like that. The Christmas 2009 version of the "smiling" Duffy came with a Santa suit sewn onto him. It was a pain to clip all the threads attaching it to him, but I managed to do it without hurting poor Duffy. Now he can be as "commando" as he wants, LOL.