I have some spec racer takeoff wheels (from miata maybe?) that I use for my track tires. they are 15x7 +35 (I wish they were 1" wider, and different offset, but this is what I have...).

In order to move them outboard a little, I bought Hub-centric spacers, which actually turned out to be really nice. I feel a little stupid though, because as I was installing them I noticed that my wheels are actually lug centric, not hub centric. I've since ordered some hub rings to keep everything nice and centered as I clamp everything together. The spacers came with long wheel studs, but I already had long studs and didn't need them.

Here are some pics of the spacers and outcome. sorry, forgot to take before pics.