The millions of white nationalists who are open about their status online, of course. That's the great thing about anonymity. You don't have to pretend, as you would on a survey. But, if you want a study, I'm sure Google can help you out. This can be looked at in many ways, from surveys that show what % of Americans think blacks are less intelligent (look it up), to more direct ones like this:
http://apps.washingtonpost.com/g/page/politics/washington-post-abc-news-poll-aug-16-20-2017/2235/
Where 9% of polled said it was acceptable to have neo-Nazi views. That number is far less than those who think the US is, and should be, a "white country".