Chris,
Sorry I didn't realize you were after a carbide bit.
I don't know if this will help you, but you might want to look up a company called "Peatol" who sell the Taig line of micro lathes there in the UK. Through them, or possibly through someone they can point you too you ought to be able to find a carbide AL4 bit. The Taig lathes use the same size tool bits as our comm lathes.