You only have two options to obtain Arca Plasmor. The first one is to buy it for 190 Platinum through the Market console or directly from your Arsenal. If you choose this option you will also receive an extra weapon slot and an Orokin Catalyst already installed.
The second option is to buy the Arca Plasmor Blueprint in the Corpus Laboratory of your Clan’s Dojo, which means that you have to previously farm the resources to be able to craft it in your foundry. Remember that this option requires you to complete this weapon’s investigation or to be a member of a Clan that has already done it.