We only have two C's: CEO and CSO (don't ask)You're kidding, right?
I was actually going to suggest CI/TO until I saw you had already mentioned "no CTO". So CIO sounds best. Unfortunately the only generally recognized titles that work have Chief or Director in them, and you're excluding both terms.
It sounds like your org is small enough that titles don't really matter: Everyone probably knows everyone else and what they really do. So in your case the title is for external consumption. I'd suggest picking the one (Director of IS or CIO) the existing chiefs are willing to live with and getting on with life.