How to create Silk Thread
Silk Thread is created by combining together two Spiderling Silk inside of a Sewing Kit or a Loom. You'll be needing Silk Thread for many different Tailoring Recipes so it's recommended that you farm more Spiderling Silk than you think you need (or buy more than you think you need).
Silk Thread (No Fail Combine)
2 Spiderling Silk
To save you some time and some clicks I have copy and pasted all of the Spiderling Silk Farming locations that I know of below.
Misty Thicket - Spiderling Silk Farming Location
The whole eastern portion of Misty Thicket makes a great location to farm Spiderlings for their silk. If you're playing on a TLP and a good aligned race this is the location that I would recommend you farm for the silk.
Nektulos Forest - Spiderling Silk Farming Location
The northern portion of Nektulos Forest makes a great location for you to farm Spiderling Silk. Hunt all of the enemies you see in the circled area on my map above in order to make room for more Spiderlings to spawn. If you're playing on a TLP and are an evil race, this is the location I recommend you farm for the silk.
Crescent Reach - Spiderling Silk Farming Location
This is another great location for you to farm Spiderling Silk assuming that The Serpent's Spine expansion is out on your server. There's a butt load of Spiderlings around the eastern exit of Crescent Reach and they have a really fast respawn timer.
The location in Goru'kar Mesa actually is better and will give you more Spiderling Silk... The only downside is the Spiders you have to defeat in Goru'kar Mesa to spawn the Spiderlings are in their 40s and are tricky opponents to take down if you don't have some gear/levels on them.
Goru'kar Mesa - Spider Silk & Spiderling Silk Farming Location
This location is one of the best locations in the game for you to farm both Spider and Spiderling Silk. The Spiders you find in Goru'kar Mesa once defeated will spawn multiple mini Spiderlings which you can defeat and claim their Spiderling Silk. It's a nice added bonus.