You can get crystals from both Hyperius and Master Gee the Invincible. Both of these fights are more tactical than complicated, and can be really difficult to do without an experienced team. Both of these bosses have quests that lead to them, after you beat the main story for the DLC.
Each boss drops ~10 crystals each. This is painstakingly low, as a weapon in the Seraph Vendor will cost you 120 shard, where as the Relic (always the Item of the Day) will cost you 50 crystals. The Relic is an upgraded version of the Blood of Terramorphous, giving both health regen and increased max health. The weapons are, stat-wise, pretty useless for the price, but can be a (slight) upgrade to the weapons you may have now.
Every one of the pink "Seraph" items have a red text, including the relic and Shield. As of yet, nobody has figured out what the red effects are.
Note that, since a recent update, you'll have to defeat these bosses on True Vault Hunter mode to get crystals. They'll be easier but won't drop crystals in Normal Mode.
Voracidous the Invincible from the Sir Hammerlock's Big Game Hunt DLC and the Ancient Dragons of Destruction from the Tiny Tina's Assault on Dragon Keep DLC also have a chance of dropping crystals upon defeating them.