Primal Mana is made from combining motes of mana and they are commonly found as drops off of ethereal type creatures like mana wraiths (elementals). There aren't really any lower level farming areas for Primal Mana because they drop most frequently in Netherstorm, Shadowmoon Valley and Blade's Edge Mountains; also now available on the Isle of Quel'Danas from patch 2.4. All of these are higher level zones. The mobs that drop motes of mana range from 66-72. These also drop in Karazhan off of a few mobs.
The best farming areas for motes of mana are shown on the maps for your convenience. The drops from the Isle of Quel'Danas are off of Erratic Sentry with a drop rate of about 40%.