As a tauren hunter in world of warcraft you buy armor in the barrens but where is the next leather armor vendor

You probably are talking about the white quality items that vendors sell. People usually don't spend money on those because they don't have any stats on them to help your character. Use the armor that drops from mobs if you can, and also use the armor you get from quest rewards. If you still want better armor, then see if there is any on the auction house available to your level.