I had been e-mailing David Kirkham about the shifter and he said they make the linkage themselves. I'd think you could send him a PM here or through
sales@kirkhammotorsports.com which is the contact us link on their website. I think the remanufactured shifter body is available other places too, like National Parts Depot. And my handle and knob came from Bill Heeley (301) 774-6710.
I don't know what David Kee has available for the mustang shifter but no doubt whatever he does have is top notch. I'm just telling you what I was told about the source of the linkage.
Or just buy Turk's, that's one stop shopping
Chuck