Any of the Seraph Guardians (from the ' Captain Scarlett and Her Pirate's Booty' DLC) have a chance to drop Seraph crystals. These are typically dropped while in the Ultimate Vault Hunter Mode. You can also obtain them through purchase from any of the various Seraph Vendors.
You can find Seraph vendors in the town of Oasis, within the Aegrus Sophisticates' Lodge in Hunter's Grotto, the Badass Crater of Badassitude, and in Flamerock Refuge.
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