The most reliable way to get Fieldrons is from Invasions and Orb Vallis Bounties. You can set up our tracker to notify you when there is an Invasion with a Fieldron as a reward.
Alternative, a Fieldron Blueprint can be purchased from the Energy Lab of the Clan Dojo. The blueprints costs 15,000 credits. Crafting the Fieldron from the blueprint then costs you another 15,000 credits, 10 Fieldron Samples, 1 Control Module, 500 Salvage, 250 Plastids and 12 hours.