???
You can get one here, that’s for sure 
well i cant buy 1 cuz my grandma doesnt wanna risk her credit card 
Why? its a secure website. tons of people buy here, why wouldnt you feel safe buying??
You could buy a pre-paid visa gift card at any wal-mart/CVS/place that sells them and then use that to buy a dark magic online.
or
Tell your grandpa your responsible and you want to learn more about money by get a checking & savings account. Have him help you set that up at your local bank and then you will get a debit card with your account. Use card to buy stuff online.