if you have been playing as long as you say, you ought to know fleet carriers are player stations...they are rarely buying or selling anything for anyones good but themselves and are a common trap.
there are no support pages externally that are a part of the game, eddb and inara are created and supported by players not by frontier.
otherwise, not abnormal to not know other basic things, or rather things that some consider basic, such as every in-game item or commodity has a market as well as at least 1 or more places said item can be collected.
common for rare items needed for engineers is mission rewards..old news and big in the forums since engineers were introduced.
then the usual various planetary and space sites for some materials. and material traders.
more to my point, if it is something that you think you can buy or sell or otherwise trade, look for its market.
in this case, if you really had no idea that missions are offering rewards that are vital to engineering, and if you have not paid any attention to any markets, then by all means use an outside source, and search to sell them, and in this case you will find Modular Terminals are wanted by many stations.
Had you paid attention to markets, or at least have a decent trade rank that you maybe earned you might already be aware of many stations nearby that want them.
That part said, go to such a station(Hint: hi tech, scientific) look in the sell section and you will see mod terminals there...defined by the game. clearly stating mission rewards.
To be blunt about the first part, anyone that thinks FC's are good trading posts for anyone but the owner and that all players are friendly and honest good citizens...isn't awake.
remove FC's from your searches on inara and eddb, you will waste less time and fewer disappointments.
I don't know what rank you have earned, but as it is common to rise in credits and rank and still know next to nothing...the game is far more complex than most seem to think.
and if you research things you will actually earn the ranks you get.
in trade knowing or not knowing how the market works can be the difference between delivering 6000t of gold for 6mi
or mining 150t gold for 50mil