At WDW, you can buy them at any guest service location in the parks or Downtown Disney. They come in denominations of 1, 5, 10, and one or two higher ones but not sure which value since I have never bought those. You can buy up to 50 dollars on a credit card and I presume it is almost unlimited if you use cash. They also seem to have limited supplies at some places so this will also limit how much and what values you can buy.