I was actually just talking to a coworker last week about this, and he told me that modern rootstock almost entirely comes from Grayson County, Texas (a.k.a. the Sherman-Denison MSA, just north of the Dallas area).

Interestingly enough, a few weeks ago I had a Riesling from Washington that had "100% vinifera rootstock" prominently printed on the label. Not every part of the world got hit by phylloxera. I understand that some parts of South America (either Argentina or Chile, I can't remember) were spared as well.