Greetings,
Crags are good for spider silks, as are the heart spiders in Guk.
I go to Feerrott for spiderling silks, as both the jungle spiderlings AND the spiders there drop spiderling silks. Helps if you're a tracking class, though!
Not sure why you need the silk threads, but if you're making padding, note that if you use LQ hopper hides for producing the padding you can use shadeling silk threads to make it, as well as normal silk threads. Shadeling silks drop from lesser shades and Xakra silkworms in Shadeweaver's Thicket. The shades also drop Swirling Shadows, which sell well in the bazaar, being used for Shadowscream armour.
Hope this helps!