They spawn in Onett Forest at a pretty decent rate (although there's always people camping, so you have to be clever).
They spawn in Deadwood too, but at a much lower rate (given that Deadwood is, well, a shell of its former self), and if they fly too far away they de-spawn).
I haven't seen any spiders, though I'm very curious about it. What happened last year? Were there spiders? If so, when did they make an appearance?