The Rubico Prime can be obtained from several ways, being the first option to buy the Chroma Prime Access, depending on the pack you buy you also receive Chroma Prime, the Gram Prime as well as unique accessories.
The second option is trading with other Tenno, which implies to be willing to pay for them, either with Platinum or with other tradeable objects, or maybe both. In any case, it is an option you can use to obtain the parts you need of this deadly Prime sniper rifle.
The third and last option you have is to obtain the Relics which have the parts of the Rubico Prime, to after open them in the void fissures and pray to have enough luck to obtain them. I may not be fast, but it is a matter of time. Is your choice.