For clothing I like the Disney store, Old Navy and the Gap (love the junk food T's though they can be expensive). I found a lot of really cute Alice In Wonderland T's for my older dd at Justice.
For other things...I either get them at the Disney Store, Amazon or Ebay.
The Gap ones are in the Junk Food brand section of the graphic t's. They tend to come and go and if you don't catch them just right online they only have the bigger sizes, they have a better selection in my Gap store.
Old Navy T's are also in the graphic t's section under Pop and Culture. Again they come and go. They usually have Toy Story and Cars for boys though. Girls usually have a good mix.
ETA: I just checked and they don't have a great selection at the moment... I saw one Cars shirt in the big boys section at Old Navy and some Star Wars shirts. The big girls and the toddler girls had several different ones, mostly princess shirts. The toddler boys only had 2 or 3.
Last year DD1 got a shirt from the Gap with Cinderella that said "Boys are nice but shoes are forever" It was her favorite shirt.
Wow, I had no idea that Old Navy and Gap sold Disney! I thought I would take the DISboarders suggestion and buy some souvenirs here ahead of time for DD, and give her something little each day. Hopefully that will save a bit of money.
Right now at Target they have Disney items in the Dollar bins (in the front of the stores at most locations) I went there and got stuff to put away to make Disney Birthday baskets for our trip in Nov. I don't think they have them online also Walmart (I know some people don't like to shop there) has some really affordable Disney stuff as well I've found shirts/PJ's on clearance for $2.00 or less. Again not online but will save you some money