Silver's purely a Silver Hand thing. Or you have to make it.
The author of Requiem hid one silver weapon in Bleak Falls Barrow, but either you are lucky and find it by yourself or you already know where to look. Otherwise you will be fucked for quite some time.
The problem with the limited supply of silver weapons is that it prevents certain characters (melee chars that don't join the Companions) from accessing an unnecessary large amount of game content - again unless you got lucky or use meta-game knowledge.
I would have made some weapons show up at least with some vendors, e.g. the ork smith in Markarth, that's supposed to be the city with the silver mines, isn't it?
It's actually one of the typical complaints I have with Requiem - at times the difference between what Requiem wants and what Skyrim can provide is a bit too large to work well.