You can obtain the Cernos Prime from three different ways. The first option is to buy the Valkyr Prime Access, where you can obtain, along with the Cernos Prime, the Venka Prime and Valkyr Prime. If you choose this limited time option you also obtain unique accessories.
The second option is to trade with other Tenno, where you can buy the different parts of the Cernos Prime you need, probably for Platinum. You can always try to trade them for other objects, although we all know that the better currency in these cases is Platinum.
Finally, you can obtain the Relics which contain the different parts of Cernos Prime, opening them in the Void Fissures. This is not the fastest way, but it is the cheapest, and that is something many Tenno appreciate.