I made this Runecloth Farming guide to help out players who want to farm the cloths instead of buying them from the Auction House. I decided to search for places with high density of mobs which I can kill and drop [Runecloth]. I traveled around Azeroth, and after a while I made a list of places.
Blackrock Depths - Dungeon
Level 40+ is recommended to farm here, but you should be at least level 35. If you don't have a high-level character, scroll down to the bottom of the guide to find alternative farming places.
Blackrock Depths is currently the best place to farm Runecloth. One clear takes about 6-7 minutes, so you might hit the 10 instance/hour limit if you clear it too fast.
To get to Blackrock Depths, enter Blackrock Mountain from either southwest Searing Gorge or northwest Burning Steppes. If you enter from Searing Gorge, climb up on the giant chain that appears to your left and go down onto the big rock suspended in the center. Follow the stairs down and exit downwards onto the chain at the bottom. The quest giver for Molten Core attunement is straight ahead and a passage leads right to get to the instance. Bear left in the cave and the instance is at the end of the chamber. Here is a picture if you still don't know where to find the cave in Blackrock Mountain where the instance entrance is.
After you cleared the instance, go outside and reset it by right-clicking over your portrait then click the "Reset all instance" button. This restarts the whole instance and all mobs will be back.
Killing Twilight Cultists in Silithus is also a good place to farm. It seems if you farm at the Twilight Base camp, you will never run out of mobs, because the respawn is really fast. But if you do run out of mobs just go to Twilight Post and move between these two camps.
NOTE: Some of these mobs currently scale to your level all the way up to 50. I'm not sure if this is a bug, or if it's intended, but despite this, it's still a really good place. If you have bad gear, you can just avoid the mobs in the middle of the camp and farm around the edges.
IMPORTANT! Silithus will be phased after completing/accepting certain quests, and most of these mobs will not be there. But, if you talk to Zidormi, she will allow you to switch between phased/low-level and Legion/high-level versions of the Silithus. (if you open the map, she will be marked with a white text bubble at northeast)
Another great place is farming Drowned Gilnean Merchant and other humanoids at Blasted Lands.
IMPORTANT! Blasted Lands will be phased in a different state after completing/accepting certain quests, and most of these mobs will not be there. But, if you talk to Zidormi, she will allow you to switch between phased/low-level and high-level versions of the Blasted Lands. (if you open the map, she will be marked with a white text bubble at north)
Visit my Netherweave Cloth farming page when you have enough Runecloth.