The smaller, 12" Duffies all come pre-dressed. I believe their hats (if applicable) are always sewn on but the clothes are likely removable. The larger, 17" Duffies come without clothes, and then you can purchase clothes for them.
In November they at least had a Christmas 12" Duffy in the window of a shop on the right side of Main Street as you come in...I'm not sure if they had others for sale, but I imagine they did. In fact, the Emporium had a Happy Birthday Duffy (12") if I recall correctly. The Christmas Duffy is very cute but I'm not sure that his mittens come off, and if they don't, I think that really limits play value. Since he's seasonal it is possible he'd be sold out by now, but I don't know.
If ANY 12" Duffy will do (and you're in America, which it looks like you are...neighbor?) you might consider ordering from the
Disney Store. They have the 2011 Hoodie Duffy for $15, which is half the price of a 12" Duffy in the parks. The hoodie comes right off and he kinda fits into some Build-a-Bear clothes. I just got mine today...his nose is a bit tilted but he's still cute. Unfortunately, most (all?) of the discount codes for the Disney Store are not valid on Duffy, but if you have a code for free shipping, that could work. Or shipping should be something like $4.50 I think, which will still put you ahead of buying at Disneyland. Unless you need the Duffy to come from Disneyland, of course!
http://www.disneystore.com/duffy-th...isney-bear-plush-toy-12-h/mp/1294004/1006801/
It would be fun to pick out a Duffy, and some of the outfits they have on the 12" ones are way cuter than the 2011 bear's (that one presumably came out in January or earlier, and they've improved designs since the initial costumes). But you've gotta love that price, and the bear himself seemed fine.
The tiny magnet and keychain Duffys should be available in that store at Disneyland too, I imagine, and most definitely World of Disney if that fails. While dining you may receive a 20% coupon on your receipt that's good 'til 12:30 PM at the Emporium and World of Disney, so if you get one and you are budget-conscious, it could be worth checking for a coupon and if you get one, waiting until Sunday to buy at World of Disney.