Did you have to specify that the pizza be made according to DOC, per Reinhart's book? As in, they have DOC pizzas and pizzas for tourists?
Pizza Napoletana is right, there was no mention of DOC (or STG) anywhere and menus were an afterthought. The only thing I tried to do was specify Bufala.
Oftentimes even with my request, they just smiled, nodded, and used what to my taste was a high quality Mozzarella - likely Fior Di Latte. Everyone seemed to be getting that in any case so in a sense it was the most authentic selection.
Personally I prefer the distinctive flavor and texture (more lactic, more runny) of Bufala so that's what I use on my pizzas at home. Cost is likely an issue too. A number of these places heap on the cheese so they would go broke if it were Bufala.
The Da Michelle pizza is the best of them all, the cheapest of them all, had the least cheese of them all (mercifully), and it looked and tasted darn like Bufala to me. They did have a 50 euro cent higher double cheese option which I saw people ordering.