Question: how rare is it to get heirloom shards?
Answer: Heirloom Shards are found in Apex Packs, which are earned for ranking up or purchasable with real money. You only have a 1 in 500 chance of grabbing them though, but with each drop your shards are added to a total. Once you reach the required amount, you can convert those shards into an heirloom item via crafting.