The only place to look for mods now is on Gas Powered Games' forums, and the forum which contains SupCom mods is the forum for mods of all games using its engine, and of course you have to dig through WIP project threads, request threads, and completely irrelevant threads on top of that, and even then once you find something interesting you might find out that the download link is broken due to the author having to resort to using not-so-future-proof hosting due to the lack of the GPGNet Vault.

To this day there is still not any good place to find mods or maps for it AFAIK, and many mods have become unavailable altogether as they had been uploaded to the Vault exclusively. Some time after Square Enix bought the rights to the SupCom IP, GPGNet and the Vault were shut down.

Supreme Commander 1 (and its expansion Forged Alliance) used to have a central place to find mods and maps, the GPGNet Vault, which was ran by the developers.