A common ways to load money on prepaid credit cards (AKA secured credit card) is to transfer money from your checking account to your new secured credit card account online. Or write a check.
The two most common ways to load money on prepaid debit cards, however, is to use a Greendot Money Pak or Western Union agent. Greendot Money Pak's are available at 7-11s nationwide. Here's some more information, as well as a 7-11 Locator.
Greendot MoneyPaks are a temporary card that you buy for $4.95, and then add cash to by handing your cash to the cashier, who then loads the MoneyPak with money. Because the MoneyPak card itself does not have a MasterCard or Visa logo, it can't be used on debit card networks. However, by logging in online or calling a customers support number, you can quickly transfer the entire balance to your prepaid card (which is why you are buying the MoneyPak in the first place.