Castelli doesn’t make it any more, but I did see a listing on Amazon for the Seamless Baselayer. I have 2 sleeveless and 1 shortsleeve and they are amazing. The mesh is thick enough to hold the jersey off your skin, which thinner mesh baselayers don’t do. Its so nice not having that feeling of the jersey stuck to your skin. They are my go to baselayers for long, hot rides.
The Core Mesh is one of their other mesh baselayers, but for me, I don’t like them as they aren’t thick enough or have enough structure to hold the jersey off your skin.