disneygiftcard.com has all sorts of info.
The giftshops at WDW and DLR sell them. disneyland.com sells several styles with free shipping.
On my son's 5th birthday we were at DLR, and he got the Birthday Fun Card, which is a limited giftcard, and it worked SO well for him. He's usually very focused on "gimme gimme", and he'll ask for the next toy while holding the one he *just* got (gets that from my husband, sigh)...but he really *got* the idea that "this is your money, spend it wisely, I'll help you do the math, but when it's gone it's gone". OK so we chipped in a couple dollars when the last thing he got was a couple dollars over the end of the funcard, but he did NOT ask for ANYTHING after that, and it took something like 3 days for him to spend it all! It was incredible.
I imagine an older kid would do even better with it. Have fun!