However, if you want to get your hands on these heirlooms, you better act quickly. Once Warlords is here, the heirlooms will be removed from Garrosh's loot table.
@Meerkatx The Garrosh heirlooms are specifically a reward for doing Flex+ while relevant. They won't be available once Draenor unlocks.- Watcher (@WatcherDev) April 26, 2014
Currently, there are no intentions at all to add the heirloom weapons to Garrosh's loot table in LFR -- as Watcher mentioned, they were meant as a reward for those doing Flex and above difficulty. And once Warlords is out and Draenor is unlocked, the rewards will be going away, so that players can't simply farm Hellscream for more heirloom items when they outgear the former Warchief. If you've been meaning to get your hands on these heirlooms, be aware that there is a finite limit as to how long they'll be available -- and get them while you still can.