You can get them through the steam workshop for now. In the future there will probably be some dedicated sites. We saw that with SC4 as well. Depends on the way the steam workshop handles large amounts of content. And the amount of low-quality and broken/cheat mods on there, since anyone can upload.
I read somewhere that TotallyMoo (CO-PR guy) intends keep a list of "good?" mods somewhere on this site.
Only 2 days and a bit now...