To obtain the Prisma Grakata you have two options, being the first one trading with other Tenno of the Origin System. You only need to find someone willing to trade it and, of course, pay the agreed price to obtain it. The good side of this option is that you won’t have to wait until Baro comes with the Prisma Grakata again.
The second option is to buy the Prisma Grakata from Baro Ki’Teer for 100.000 Credits and 610 Ducats. But you should stay alert and be prepared (to take the chance) because Baro’s stock changes with each visit.