Runecloth

I've went through and checked pretty much every suggestion on here. Most of them are out-dated and not up to date with Cataclysm, in which case they don't drop runecloth.

As of 12/28/10 (patch 4.0.3a) the best place to farm runecloth is BRS (Black Rock Spire).

When the orc mobs drop runecloth it is usually between 2-4 (very often 4). The best thing to do is go inside and clean out the very first room, then go up the stairs and clean out the large room. There are groups of orcs (4-5) standing together in single rooms which go down the length of the sides of this large room. If you have a good AoE spec and high enough level you could probably pull the entire room or most of it, which would be like 30-40 orcs give or take.. All of which have good possibility to drop 4 runecloth each. Yeah I would say this is the best place to farm for runecloth.

After you clean out this large room the best thing to do is leave the instance and reset it, then do it all over again. You will be very close to the entrance (like a room away), which makes this have huge potential for farming runecloth. Basically you can farm it as fast as your character is able to.

(The only other place I found that still was farmable on Cataclysm for RC is Sunken Temple and it doesn't compare whatsoever to BRS.)

Short Version: Send your pet to pull the entire Hellfire Ramparts instance for you. Then have him drag them all back to the entrance.

When you zone into Hellfire Ramparts, you can see the end of the instance above you and to the right. This is normally the shortcut after completing the instance that lets you jump down and return to the entrance. But it also means you can direct your passive pet to Move there. As he does, he will aggro the entire instance. Once he's there, tell him return to our side. He will run back, pulling anything that was missed along the way. It takes about 60 seconds for him to run to the end, and 60 seconds to run back.

You stand at the start of the instance, and wait for all the mobs to arrive; where you kill them with a Multi-shot or Barrage.

Your pet does all the grunt-work, leaving you to do more interesting things, like write about it on wowhead.

All of the above spots are okay, but the one place that worked best for me was Silithus grinding the Twilight Mobs. Mobs have around 3.8k health and are easy kills, packed closely together and most drop around 1-3 runecloth.

They can be found here:
http://www.wowhead.com/?npc=11880

I'm not the quickest grinder (no AoE) but I got about 3 stacks in 30 mins.