A rat NPC in Nashmu sells there for a little under 5k/stack. They are also made at 17/18 Alchemy, with Light Crystal + Polyflan Paper + Mercury + Elemental Cluster(Light Cluster for Light Cards, Ice for Ice Cards, etc.)
They are used at level 40, when Corsairs get the ability, "Quick Draw" These add elemental damage to a Corsair's bullets, and are rumored to enhance enfeebles/nukes of the same element.
They stack to 99. Corsairs can cary as many as they want, like Ninja tools. 1 use/card. Each NQ synth makes 33.