In demon souls, the chrystal geckos in 2-2 disappear and don't reappear. where else can i get hard stones?

The splinters and large splinters are relatively easy to get, just buy them from the filthy man in 2-2, they are 500 and 1500 souls respectively. The chunks are harder to get. You can get those from the miners in 2-2, but again, they are rare, so good luck would help. The pure hardstones are extremely rare, and you can only get those from crystal geckos and from one above the nest you mentioned on a corpse. Sometimes if you wait a while, the geckos respawn. And there are geckos in other worlds too, not just 2-2.