You can buy Disney Dollars from any
Disney Store in the US, or from WDW Mail Order - (407) 363-6200, or from Guest Relations at any of the WDW parks or resorts in the US.
If you're not married to the idea of Disney Dollars, you could also get her a Disney Gift Card, which works a lot like a WalMart gift card - no fees, no expiry, reloadable, minimum initial load $5, good for use at WDW, DLR, Disney Stores,
Disney Cruise Line, and DisneyShopping.com. You can get them from any Disney Store in the US, at most of the shops and stores at WDW or DLR, and online from one of two places:
Disney Gift Card at Disneyland.com
Multiple designs, only available in initial loads of $25, $50, or $100
Disney Gift Card at DisneyShopping.com
Only one design, available in initial loads of $25, $50, $75, $100, $125, $150, $200, $250, $300, or $500
*Note: All of these are Disney Gift Cards. They can be bought from multiple places, but they are all the same card, on the same system, and are all accepted at all of the locations I listed above - there are no more gift cards good at only one Disney location!
**Note: Although the initial load amounts are preset when you buy online, you can take cards bought online to any Disney Store and add money to them in any increment you like from $5-1,500. Initial load when bought at a Disney Store is not set - you can load with any amount from $5-$1,500.