Would anyone know where I could get hold of a Shimano compatible Lightweight freehub? I think but I'm not sure that the freehub used by Carbonsports on their standard tubular rear wheel hub is a DT Swiss Onyx...

Do you just need a freehub to replace a current Shimano freehub or are you converting from a Campagnolo freehub body?

If you just need a replacement, you need a DT 240 freehub. If you are converting, you need a DT 240 freehub plus a new end cap for the drive side as it is different from the Campagnolo one.

Either way, it depends on where you are as regards where to buy one from. In the UK, the usual suspects carry them (Wiggle, CRC, and I know that Sigma Sport and Slane Cycles also do them).