If you purchased a Founder's Pack tier that includes friend codes, you can find them in the in-game main menu. Simply click the hamburger menu in the upper right corner and you should see the Friend Codes! option in the second column from the right. Please note that these friend codes will not be available until you have completed the Loot Llamas tutorial mission.
Friend codes are not region locked, so you will be able to share them with friends in any region where Fortnite is available.
PS4 and PC players: Fortnite Standard Pack Friend Invites must be redeemed on the same platform the originated from. This means that PS4 invites must be redeemed on PS4, and PC invites must be redeemed on PC. After the code has been redeemed and Fortnite has been launched at least once, your friend can enjoy cross-progression on both platforms.