d12e8b4e_sic_Dome_projection___Domerama__Spherical_mirror
[0] Cost and availability [Fisheye] Expensive, not always available for all projectors. [Spherical mirror] 
A quality spherical mirror for dome projection can easily be found at reasonable cost. Multiple sizes are available. For example a quarter mirror between 9″ and 18″ varies from $125 to $200. They are also available as used.
Geodesic Dome projection » Domerama
Spherical mirror