Like most Prime weapons, you can get the Boltor Prime from three different ways. The first, and the most expensive one, is to buy the Rhino Prime Access which, besides Rhino Prime, it also includes the Ankyros Prime. Remember that this is a limited time option and, apart from these three Orokin pieces, it also offers unique gear.
The second way, as always, is trading with other Tenno. There are those who are good at this and obtain rare parts at best price, or can make the most of what we could call “collector’s parts”. In any case, you can get the pieces to complete the Boltor Prime trading, which implies being willing spend Platinum or offer gear interesting to other Tenno.
And the third one is farming the Relics which contain the parts of the Boltor Prime, and have enough luck to obtain them when you open the Relics, something not frequent by the way.