Eventually they will all be located and supposedly easily installable on steam workshop. In the mean time (it make take a bit to get thatall working just right), you can find and install mods manually. Go to the mods folder on the forums, thats usually where peeps post their mods. Or just do a search on forums for "mod". A LOT of threads will contain them, you can tell from the title.
To install a mod, you need to place the high level mod directory in the ..\MyGames\GalCiv3\mods directory. The mod directory installed should be at the same directory hierarchy as the "ExampleMod" directory (this in fact is a dummy mod folder, with ModdingReadme file which explains the same thing I am doing now.
However, some mods actually require you to (or they ask you to) patch the full game directory (under steamapps if you have steam). I dont see why this is necessary, as the whole purpose of the mods directory, is that if there is a file in the mods dir when booting up, which has the same name as a proper file in the full game directory, the mod data supersedes the original data. Anyways, any good mod writer will have an installation instruction with it, especially if it has exceptions.
Oh, and the most import thing after you put the files there, is to launch the game, go into options, and ENABLE MODS (off by default). Else, the mods directory will not be read. for now atleast, you can tell if you have mods activated or not quickly by seeing that multiplayer is disabled. MP wont run with mods atm.