The best place to farm regular spider silk are the blackwidow hatchlings in Duskwood. They are just east and south of the Ravenhill Cemetary in the western portion of the zone. Watch out for Stitches, a 35 elite mob, that infrequently spawns nearby and runs through the area on his way to the road and local human town.
Thick spider silk is not worth nearly as much as the regular kind, so buy any of the thick that you might need instead of farming it.
Edited, Aug 27th 2007 3:47pm by ItsaGaAs