While Tamron has made one since day one sigma don't make one, Chinese versions can be found on eBay for both now.
Sigma
https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=sigma+100-400+tripod+mount+&_sacat=0
Tamron
https://www.ebay.co.uk/sch/i.html?_odkw=sigma+100-400+tripod+mount&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=tamron+100-400+tripod+mount&_sacat=0
Sigma
https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=sigma+100-400+tripod+mount+&_sacat=0
Tamron
https://www.ebay.co.uk/sch/i.html?_odkw=sigma+100-400+tripod+mount&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=tamron+100-400+tripod+mount&_sacat=0