You cant really grind them. You get them from clan boss chests, you can buy some with gems. You also get them weekly for doing your daily quests and now you can pick them up from cvc

But the biggest source is via credit card or waiting a long time for the above to come around.

Once you can do Nightmare clan boss you will get a steadier source of shards, ultra nightmare drops a fair few sacred shards (i got one today)

Grind any dungeon/keep except minotar

Hello! I've got another question! What is the minimum to getting shards in CB because the maximum I'm able to do is Normal, which is already a challenge for me.

Ancient shards have a chance of dropping from all levels of cb. if you can get the top chest on normal, it also has a chance of dropping void shards.

Oh yeh i had forgotten this, sometimes dungeons drop a blue too.

I didnt know that, i dont think i ever had one drop for me below nightmare.

You can check now, if you click on the chests it shows you the possible drops. And pretty sure I did get a void shard from a normal chest once back in the day. But the odds are very low - probably below 1 percent. So obviously you'd get far more of them on the higher difficulties. Similar to how the top NM chest can actually drop sacred shards and lego tomes but almost never does, you'd need the top UNM chest to get them with any regularity. 

Someone else had linked this but below is the chances of getting a shard from CB (credits to Bobo for building this).  You can right-click on it and choose to open in a new window to make it easier to read

As you can see it is pretty low to get an Ancient shard from anything below Nightmare, but it is possible

i

ancient shards are 0.50% chance across all levels of dungeons/keeps

CB has a sliding scale per increasing difficulty

Thanks!!

What is the chance of a rare appearing bc I've gotten one a couple of times.

Getting Ancient Shards is supposed to be difficult. Because there is a chance of getting a Legendary, there is no straightforward way in the game to grind Ancient Shards. 

Apart from what is said by other participants of this thread, regularly checking Market might be a good idea - some amount of Ancient Shards pop up there now and then. Weekly quests are also a slow but steady source of Ancient Shards