Mana comes from the mana mines. Deep underground caverns where Gnome slaves labour away relentlessly, chipping away at the walls for mana crystals. These crystals are then eaten by the Gnomes, and the resultant processed product harvested from their faeces is mana in the usable form for spells.
The shards were formed when the Titans developed chronic constipation, as they used to process their own mana crystals. Unfortunately, the shard defecations caused all the Titans to implode due to the internal vacuum created by the exit wounds and the air pressure created by the rapid outflux of stinky magic. That is the real story behind the disappearance of the Titans that no one wants you to know.
See, even fictional worlds can have crackpot conspiracy theories. 
