You can get them at the parks. I would go to the main stores in the parks...the smaller ones don't seem to have them. And even those stores tend to have to search for them (under the register's cash drawer was where the CM found mine).
But I don't see much point in them. Then again, ever since wamu was "bought" by Chase, "my" bank has ATMs all in the parks, and I don't have to worry about bringing cash, as I can just go GET cash. And suddenly needing cash was the reason I bought the DDs in the first place, back before the moving carts in DCA started taking cards. My family was off elsewhere, I saw it as a good chance to have a Mickey Bar (forbidden to DS and DH b/c the ice cream has corn syrup, which they cannot have), but had no dollars...scramble scramble, and then...the cart closed by the time the CM found the dollars.
But now Chase is there so I don't have to scramble, and most of the carts take cards, so...no point. For me, at least.
For the DTD restaurants, you could always go and ask (or call to make reservations and ask?) *before* going to buy the DDollars. On the one hand, they can be turned back in at Disney giftshops for real money, but on the other hand, why would the management at those restaurants want to do that?