Chunks of spider stone are fairly rare, and needed in sizable quantities to upgrade down the Sticky upgrade path. They drop far less commonly than Shard of Spiderstone from the unarmed miners, and a Pure Spiderstone is guaranteed at the 2-1 Armor Spider Archstone after defeating the Armor Spider--the limiting factor when it comes to upgrading bows down the sticky path almost inevitably becomes chunks of spider stone. The easiest way to obtain more chunks of spiderstone is to kill the crystal lizard in 2-1 at higher NG levels and hope for the best.