Here's a stupid idea: 18" diameter wheels with black lip to simulate 15"!

17" steelies maybe? I couldn't find 18" steelies but those might look pretty tough as well. Are you just wanting a better compound or are you looking to widen things up in the front? 18s should clear your tie rod end, and there are some guys running 275s on the front with those (I was too nervous for that, 245s for me).

http://www.deluxewheels.com/the-wheels/ makes essentially what you're talking about in a whitewall, but those only come in 20" which imo is too big for a dart (and I have 18s, which everybody already says is too big). The prices are also insane.

I don't know how you use your car, but if you're looking for just some extra performance for auto-x track days (or similar), you might be almost as well off getting a decent quality set of track wheels (Enkei RPF-1 is kind of a go to for a cheap, light wheel) that you don't leave on all of the time.