You have several options to obtain the Nagantaka, being the first one to buy it through the Market Console or the Arsenal of your ship for 125 Platinum. If you choose this option, besides the weapon you also obtain an Orokin Catalyst already installed as well as an extra weapon slot in your arsenal.
The second way to obtain the Nagantaka is to buy the Blueprint in the Market Console of your ship to craft it later in the Foundry. To do that you need all the required resources. This is the slow option, but is the most affordable and popular among all Tenno.