Amazon also has it through a reseller - "Wrapables." Interestingly, when searching Amazon for "YINSH" it didn't show up until result 7 or 8. It's pricey, though, at $39.95.

It's also available directly:

http://www.wrapables.com/jsp/Search.jsp?Key=yinsh&Category=0...

And, Thoughthammer.com has it listed as a preorder for Q4 2011 for $21.42. Maybe it's out of print but not for long?

Edit: YINSH is scheduled to be reprinted in Q4, 2011, according to Jay @ RGG.