I've recently came across the artist Isamu Noguchi (wow, what a
talent!) and would love to know where his sculpture Undine(Nadja) is
currently being exhibited or if I could get a poster or post card of
it.

All that I seem to turn up in my own google-ing is this photograph:
http://www.noguchi.org/inwundine.htm

Thanks in advance!

Bad news for you. I emailed the Nogucki Museum in NYC (recently
reopened) to inquire where Undine might be today. Here is what this
pretty authoritative sources had to say:
"I am afraid Undine, as far as we know, is no longer
extant.  Our research over the years has not turned up any information.  If
you ever come across it or a reference to it, kindly let us know. We would
certainly want to know.
Best wishes,
Bonnie Rychlak
Curator"
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Thank you Scribe. I had a sinking feeling that the evocative sculpture
is MIA when I couldn't find any mention of it beside the photo. thanks
for letting me know :-)