From what I see, they're a bit more rare to find than the other spawners. Keep looking, you'll find one eventually c: (or you could look through /warps and see if anyone has some for sale)
They are honestly not that hard to find. Besides, some shops sell them for a cheap price. Some people also keep some laying around somewhere in your home. Once I saw someone auction a skeleton spawner for only $5 due no one bidding on it. Some people really want to get rid of them.