The community 'Bugfix' mod for Soulstorm is still current at ModDB, version 1.56a (and is definitely a must have mod for Soulstorm):
http://www.moddb.com/mods/soulstorm-bugfix-mod/downloads/soulstorm-bugfix-mod-version-156
The only other mod I'm using is the Dawn of Skirmish A.I., which I used to get from the Revora forums, and apparently it's still there:
https://forums.revora.net/forum/478-warhammer40k-dawn-of-war/ (lots of sub forums, and specific threads too when scrolling down from that page)
https://forums.revora.net/topic/72022-dawn-of-skirmish-ss-v320-ai-mod-is-now-live/ (still using version 3.20 myself, and works in conjunction with the Bugfix mod too; I don't think that DoS was ever updated past 3.20, however)
Now, when it comes to specific modding communities outside of ModDB, and to some extent those Revora forums, I wouldn't be aware if there's more (known, named similar modding communities for Soulstorm or Dark Crusade specifically). When it comes to mods / modules, or custom maps I normally just go on Google and search for something like "Soulstorm maps", or "Dark Crusade textures" and so on, and just check the first couple of results and get my stuff that way (and of course ModDB often pops up among those first few results). But I haven't done that in ages (last thing I got was the latest Bugfix version at ModDB, but that's it).
I've been using my custom maps since the Dark Crusade years (which also work for Soulstorm on a side note) and all the other mods I used to have (but don't have anymore) came from other modding communities / sources which I don't remember the names of, or web sites (or networks) like the famous (and infamously gone) FileFront. After FileFront closed I recall a brief period during which a lot of the mods for specific games were migrated to other networks or newly-created private web sites to keep those mods active and for would-be modders (and teams of modders) to keep working on them. Many years passed and I haven't followed anything regarding the modding community aspect of our beloved Dawn of War, Winter Assault, Dark Crusade and Soulstorm games since then. So at this point, indeed, the only one that comes to mind immediately is ModDB. I suppose, now that I think about it, that ModDB is pretty much the only equivalent to FileFront we have nowadays.