The measure you're looking for is the Abbe number. Many opticians won't know what you're talking about, but if you push and/or research materials yourself this number is readily available. I remember comparing materials with (IoR-1)/Abbe as my "badness" score, but don't cite me on that.

A while ago I surveyed the materials that were available. The usual 1.6 proprietary materials (Sola/Hoya/Shamir/Signet) have Abbe of 42. The 1.67's are 31 or 32. I did see a reference to a single Zeiss material with an IoR of 1.67 and Abbe of 42, which you would be interested in. I'm sure it's going to be expensive though.