The Bronx Zoo does some great work with tortoises and turtles but the majority of the species listed in that article are all kept off-show The only reason I've seen the Forsten's and Yellow-Headed Box is because they zoo temporarily displayed them in their reptile hatchery area.
The same is true for Munster Zoo - Germany - which also has a huge collection of realy threatened turtle-species but also most housed in a breeding-area not open to the public.