Yes, you can and you don't have to be 18+ to buy them, as they are not loans. They are very good tools for teens to learn money management. My wife and I gave our kids one of those and replenished them, instead of giving them cash for pocket money.
Go to your bank.... and open up a VISA DEBIT account...
Then you just put in as much of your own money as you like.. thats exactly what I did.. I don't want a credit card but I wanted to be able to access services online so I got a Visa Debit.. Pretty much every bank has them. I'm now loving shopping on the net with my visa debit. Good luck!