e-bay. They pop up. The double CD with the 95 and 78 cast recordings together seems to be easier to find than just the 78 single CD.
You could also try:
Dress Circle Records in the UK
http://www.dresscircle.co.uk/or
Footlight Records in NYC
http://www.footlight.com/or
Sound Of Music out of Germany
http://www.soundofmusic-shop.deAll 3 specialize in rare cast recordings and soundtracks. I looked and none of them seem to be carrying the CD in stock anymore (although all 3 of them did stock it when it was in print). However, they may be able to hunt one down for you, for a price of course.