I quit WoW about a month ago, still miss it from time to time. So I guess I'll just resign myself to browsing forums .
The best place for Mageweave, Horde side only, is Nethergarde Mine[Blasted Lands(53,12)]. Its great because the mobs are friendly to Alliance. I picked up alot of Mageweave there when I power leveled my lock's tailoring.
I just ran SM solo for silk. DE'd the BOP blues from the bosses, the Small Radiant Shards sold quite well on my server.
Thottbot is a great source to see what drops where. As mentioned above, the SM instances seems to be a good source of silk for example: http://thottbot.com/i4306