>>12527730This, but you need to rely on luck. Antiques will sell rocks and boxes who contains random items inside. You can get Shards, Big Nuggets or just crappy items like Pokéballs and Hard Stones.
If you need shards, I suggest that you go to Relic Passage, use the Pass Power that increases the chance of Dust Clouds and run like crazy after the clouds. You'll get something like 6-8 shards in average in 3 minutes (The duration of that Pass Power).
You can also enter the PWT and quit without fighting, you'll receive a Shard as participation prize, but I think this way is slower than farming with the Pass Power.