They do make spacers. They're called just that. But first you have to know if it's a positive or negative offset. I never can remember which way is which, but one's got the bolt plane (or whatever it's called) closer to the outside of the wheel and one's got it closer to the inside. If it's closer to the outside, and the wheel is of a lower offset, you can't get a spacer because you'd need to remove material, not add it. It's not clear which way you're going, though.

Also, I'm not sure that spacers are street legal.