I've never seen a tri wetsuit rated in millimeters. I have seen it for surfing wetsuits.

Tri wetsuits are made for swimming and the thickness varies (thinner in places that allow for more movement when you're swimming.) Does your local shop have any wetsuits made for tris?

I have an Ironman wetsuit (which is now Blue Seventy). Other companies that make tri wetsuits are Quintana Roo, T1, Orca, Zoot.

Mine is a sleeveless, so it's a little easier to get off, and I've worn it in temps down to the mid 50's, but I"m pretty tolerant of cold.