15" Steel Rims on a '68 Dart

Only the A-bodies ever got the small bolt pattern, and there was never a 15incher among them.....So yes, you can forget about ever finding a factory one.
You can have a local guy make them, or the better idea is to call the wheel guys and order what you need.
OR
AFAIK those poverty caps also came in 14s so .......... it would be a lot cheaper to just get the smaller version, just saying.