The only repeatable source of shards in Omega Ruby & Alpha Sapphire is going to be various Mirage Spots. All the other locations are 1 time only.
Here's a detailed tutorial on every shard location, and you'll see the only repeatable options are these Mirage Spots:
Red Shard = Mirage Caves (any)
Blue Shard = Mirage Caves (any)
Yellow Shard = Mirage Mountains (any)
Green Shard = Mirage Mountains (any)
The shard will always be a hidden item and there's only 1 hidden item on the 1 mirage spot available each day. But if you keep checking daily with the Dowsing Machine you'll be able to acquire as many as you want over time.