Shadow Silk

Quickest way I have found to farm these, is LBRS: Lower Blackrock Spire.

If you have a decently leveled mage alt, that is the alt your wanting to use.
(Though anything with an ability to avoid fall damage works well too. I just prefer having Blink for faster run times.)

Once inside the instance, a quick left, left, and right, gets you to the main room. This place is huge, but we are only wanting to get to the Spiders that drop Shadow Silk.

Directly to your right, you can look down and see a lava river and some huts at the very bottom. Using Slow Fall gets you down here both safely and quickly. By passing all these grunt orc mobs you'll see a shadow-y corner with Spire Spiders inside it. These you'll want to kill for the Shadow Silk.

Now you are probably thinking...Ugh. I'm all the way down here. I really don't want to have to walk all the way back up just to reset to get more, right? Easy enough. Following up the path you'll see some Spirestone Warlords and Battle mages, sticking to your right you'll come to a ledge that drops back down to where the grunts were before, where you had used Slow Fall to reach. This time, instead, just drop down and lower your health. As a 110 mage, this dropped my health to about 5%. Take a quick dip into the lava, and you'll die fairly fast. (If you want to avoid repair bills, feel free to use the equipment manager and go naked. Though, this is purely your choice.)

On release, boom, your back at the entrance of the dungeon. Exit, Reset all instances, head back inside, and repeat for more Shadow Silk.

Lower Blackrock Spire is a great spot for this silk. Spire Spiderlings located at Mother Smolderweb drop it and in decent amount. I've averaged around 3 to 4 silk per clear and once it was even 6. It takes at maximum 2 minutes of travel by foot without any speed buffs from entrance to Mother Smolderweb, if you take the scenic (normal) route.

You can shorten the time by jumping down from the first platform from which you can see the ogres when you enter LBRS. Then just pass the Firebrand orcs and continue to the spiders. Total time is like 30 seconds.

I recommend checking out LBRS.

Common drop off any 50-58 spider, as the level of the spider decreases so does the chances of the Silk dropping. It's used in tailoring to craft the "Shadoweave" pieces of gear.