As it sits right now the shard drops are reserved for special events (based around holidays generally) but the legendary dragons are technically paid content so to rank them they want you to purchase the shards from the store
Yep as Godsillest rightfully said, you will have to wait for the "special events" for legendary dragon shards. But do not be expecting vast amounts of shard in those events unless you are going to get LB ranks. So far I have participated in every event and have a legendary at rank 8 ( F2P player in events ) you don't get many shards unless you're paying, but the legendary is still stronger than the Salty ATM (these shards seem just as rare and hard to acquire as legendary). In general I think you really have to pay to get the most out of any legendary
I'd still love for the shards for the 2nd set of PvP dragons to be available before we have to go into the 23+ bracket...especially if we have those dragons already, but are only like TH20. They should go into the rotation when we get them...