Have a look at the Heinie DOJ w/thumb-break at http://www.heinie.com These are made exclusively for Heinie by Lou Alessi.
Unless you are absolutely set on a thumb-break, you may want to consider Alessi's Belt Slide Unit with pull-thru straps. These provide a mechanical snap latch, but yield to the drawstroke. You can see them at http://www.rfholsters.com
Mark Waldon http://www.waldonholsters.com also offers several holsters with thumb-breaks. I can vouch for the quality of both Waldon's and Alessi's products.