According to her homepage, the most recent exhibition ended in January and the next one is scheduled from July to September up in Ueda City, Nagano:
That means (1) you'll be waiting a while, and (2) you'd have to find someone who lives in/near Nagano, or is enough of a Higuchi Yuko fan that they'd be willing to make the trip, which strikes me as something of a longshot. (I wouldn't hold my breath waiting for them to be officially sold online; the way they're described on the website says they're specifically designed as exclusive goods for her exhibitions.)
If you really want the cards, I'd say that Amazon is your best bet.