Why Can't I Find This

I need an arbor that is JT6 on end and JT3 on the other.  There are gobs of these from one MT to another, and many to JT3.  But I can't find anything with JT3 on one end and JT6 on the other. There are morse sleeves to convert Mt1 to MT2.  I could use something like that to adapt MT2 or MT3 or MT4 to JT3, but even these are not there.  I have emailed Jacob's Chucks in the hope thy have them.