Hi Blaise,
I have the same problem. Found several companies on the web who produce plastic or acrylic spheres with various diameters but the problem is that they only ship large orders. I can give you some links if you contact me at bmaedler@physics.ubc.ca.
recently I found acrylic hemispheres in a art supply store (something like Michael's). the largest diameter is 12cm and you would need to glue the hemispheres together and of course also fill them.
The latter seems to be the biggest challange. To release all air one has to fill the sphere completely. That makes sealing the remaining hole a problem. Also due to temperature contraction and expansion these spheres tend to leak over time.
To order some type of spherical phantom that might work for you, have a look at:
http://www.phantomlab.com/magphan.html

Good luck and let me know if you find something else.
Burkhard