Here's a place that will sell you just one for $2.90/Ea. http://www.tapplastics.com/shop/product ... e5d34a6c7f
I have ordered from them before, as was very pleased with their product and service. The balls were crystal clear.
Also, I ordered a bag of 10 for $25.50 from this next place about 15 days ago. http://www.jmkdisplays.com/shapes.html
Product # ACRB-150 1.5" BALL $2.55/Ea.
I haven't received them yet, and haven't ordered from them before. I understand you only want one, but maybe a call to them could convince them to sell you only one.
As a last resort, I was able to buy a few from Tannens last time I was in there a few years ago. They had bucket of them, but the quality was not as good as Tapp Plastics.