Very tough. There are thousands of sites that feature the quote, but none that say where it's from.
I've searched through his collected works and on several poetry sites. It's not there. It appears to be one of those things where "everyone knows" Yeats said it, but no one can prove it. Probably that means either that it's fictional or that it was first published in a newspaper interview or something of the sort.
Tough question. One of these sites may help you.
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