If it's just a spare, I've got a 15x7 that would get the job done...unless you're running 15" wide tires, I'm with Snitty on this one....why so dang wide?!?
Just bought a used '01 Sport that is running 35x14.50R15s Currently have no 5th wheel to mount a spare on. The installed wheels are all 15x12s or so I was told. Seems to be right considering the size of the tires.
I think I've decided to just buy a whole set of (5) 15x10 wheels; gives me more options for tires anyway.
I'd say stick with the 10s... I can't help you with getting them. But I believe they will be better for you if you ever decide to get smaller tires for whatever reason.