I have found packs of Disney themed paper at Michael's, Hobby Lobby and Archivers. In fact I now have too much Disney themed paper!
The packs usually come in 'books' in which you have to (carefully) tear out the paper. You can find them in sizes ranging from 8x8, to 8x11 to 12x12 in styles including Magic Kingdom papers, Princess papers, and Mickey papers (including "Simple Backgrounds" and "Mickey and Friends"). If I recall properly the non-sale price of 12x12 books of Disney paper was $9.99 at Michael's.
They can also be purchased at Paperwishes.com. Go to their site and click on the tab for Disney or do a search for "Disney" and you will get 8 pages worth of results of paper and other items either officially Disney or related to Disney. They sell the 12x12 packs for $8.95.