The Nikana Prime can be obtained from three different ways. The first one is to buying the Saryn Prime Access, which includes the Nikana Prime and also the Spira Prime. Moreover, you obtain unique accessories if you choose this option. Keep in mind that this is the most expensive way to obtain the Nikana Prime, Tenno.
The second option is the trade with other Tenno. This is an option which many use occasionally, because it is a fast way to obtain the parts you need. However, remember that the price will depend on what they want to ask you for and on your trading skills.
The third and most traditional one is to obtain the different parts opening the Relics in the Void Fissures. Here you will depend entirely on your patience, effort and luck.