Xbox live for fortnite

Hello I recently decided to cancel our 14.99 live membership for my son to play Fortnite. i feel it is to expensive to have to pay that much for him to play. Reading all these threats I noticed I can buy just a GOLD membership for 3-6-12 months. Where can we have access to that? I havent been able to find those prices in microsoft.com.

thank you

:)