Yes, you need to go to the online Apple Store
LINK and purchase the iTunes Gift Cards
LINK. Other formats of gift certificates etc are explicitly excluded from the agreement.
Use your iPhone credit to pay for the gift cards. When they arrive in the mail, APPLY the gift cards to your iTunes account.
I suggest you order 4 x $25 RED cards and then you'll also be contributing to a great cause. (If that's your bag)