You can find Heirloom Shards inside Apex Packs, earned during the level-up process or by paying with real money. You have a 1 in 500 chance of getting Shards from an Apex Pack. Also, the developers designed it so that you loot one when opening the 500th Pack. In the end, you can combine Heirloom Shards into an Heirloom by crafting.