To obtain the Pyrana Prime you have three possible options, each one with its pros and cons depending on how you look at them. The first one is buying the Limbo Prime Access, which includes both the Pyrana Prime and the Destreza Prime. If you choose this option, despite being the most expensive one, you also get unique accessories included in this Prime Access.
The second option, and already a classic one, is to buy the different parts which compose the Pyrana Prime to other Tenno through trading. The positive part is you can quickly obtain the parts you need. The “not so positive” part is the price to pay will depend on what they want to ask you for and your trading skills.
And finally, the most traditional way to obtain the different parts of the Pyrana Prime is to obtain first the Relics which contain the parts, then enter in the Void Fissures and open them. Of course, the success of this option will depend mostly on your patience, perseverance and luck.