Spawners don't necessarily come with a mob type, though they do cost 2-3 diamonds. Once you get it, place it and have a donor change the mob type to Skeleton. :3
Yeah, if it's in a dark area, it will. You can have someone change it after that if you wan't, but you can't pick it up after it's been placed so be careful.
I can sell you one, but i am selling it for 4 Diamonds, since the demand for spawners are pretty high right now.