I know it won't be 15 but I'm sure you could find a 16", The Dodge Charger/Magnum shares the same bolt pattern as us, I'm not sure about the offset, just a thought.
EDIT: had another thought. I assume the steelies will be drag only right? If so, have you thought about widening them? I've seen it done for four-wheeling and I've seen them make them thinner for the drag cars. Might want to check in that.
Another option is to get four steelies and weld two together and get some really large slicks.