Like anything else, the sources have dried up and they are selling for the second or third time on the secondary market. The advent of NC Star stripper clips was really unfortunate. Recent vintage Chinese made but of lower functionality. Many/most are 31 stamped like some of the authentic military surplus Chinese stripper clips.
The best bets are the Yugoslavian, Russian, Czech (bxn), and old Chinese if you can tell the difference. The Chinese stripper clips came from many different arsenals besides 31, so those others are sure bets too.