ksj8ak2: You will get the Blueprints for the 50, 100 and 200 tonne Frigate Fuel automatically, once you activate the console in your first Fleet Command room on your freighter for the frist time (or was it, when after activation you spoke to your Navigator ? Can't remember exactly). There is no need to collect Salvaged Frigate Modules in order to unlock the Frigate Fuel blueprints.

As for obtaining Salvaged Frigate Modules, I found one in the four or five Crashed Freighters I checked so far. But since I usually ignore the Crashed Freighters, the majority of Salvaged Frigate Modules is brought in by my Frigate fleet. I've had one Mission bring in three at once.

Also, patch 2.09.3 finally reduced the obscene amounts of SFMs formerly required for unlocking the Freighter Hyperdrive Upgrades at the Freighter Research Terminal: 200/400/800 to 4/8/12.

Oh, and for the materials required for creating the actual Frigate Fuel, I usually just buy Tritium and Di-Hydrogen Jelly, which I then refine into Di-Hydrogen.

The Debriefings of the Frigate missions can be a hilarious read with their sometimes dry sense of humor, or descriptions of how they got their a** handed to them :D

Edit: I allways checked the Manage Fleet terminal for Frigates waiting to be Debriefed in order to check for finished Missions -- until I noticed, that the Frigate holo-models in Fleet Command rooms stop moving, once they're back.

XenonS: For the first time I got one of these modules in a crashed freighter container. Scan with your visor to see these containers and mark them. What's inside in these containers is random each time, but at some point you should get a Frigate Module.

The description of the module says that you can find them in crashed freighters (sic) and after the return of successful frigate expeditions.

You can also get directly the upgrades which these modules are building: kill sentinel ships in space.

Good luck!

ksj8ak2: Another way is to pirate them from freighters in space, instead of putting up a fight with System Authority. Just make sure you exclusively hit the freighters' cargo pods, and not one of the ships floating around it in order to not lose rep.

A single System Authority ship will pop in and start shooting at you, but as long as you don't attack back, no others will spawn.

If you have enough, and are too far away from a Space Station to hide from the authorities, just summon the Anomaly and slip in.