I do beleive that disney dollars can be used at the
Lego store...I remember being told that after we were told we couldn't use the giftcard.
I do not know about other DTD stores other than WOD. But you can't order them online, as far as I know, or any way...remember, they are exactly the same as cash at Disney, so sending them in the mail means if they are lost they are absolutely gone.
I would make up my OWN dollars (If I were going to do this), be sure to put the real dollars aside, then once you get to
Disneyland, go by some at a main giftshop. Then trade them out with the kids.