To obtain the Komorex you have two options, being the first one to buy this Sniper rifle in the Market Console or through your Arsenal for 225 Platinum. If you choose this option you also receive a weapon slot for your inventory and an Orokin Catalyst already installed.
The second option is to buy the Komorex Blueprint in the Corpus Lab of your Dojo. To do that you must have completed its investigation. Once you have the Blueprint of the Komorex you can craft it in the Foundry of your ship. As you can imagine, this is an option where you need to invest some time and effort, but it is also the classic choice of the Tenno.