I need a "Lightweight High Pressure Belt Drive Single Stage
Centrifugal Blower" which delivers 3,000 CFM at 6.25 psig that is
light weight.  I do not need the electrical motor and stand that these
are typically packaged with as this unit will be driven by a gasoline
engine.  Because of the gasoline engine drive, I have very wide
latitude on the input driving speed, from about 3,000 RPM to at least
12,000 RPM and probably higher.

Light weight is the most driving issue.  I have some latitude on both
the flow and the pressure, say approximately +/- 10-15%.

Where can I get a compressor like this?  If I can not get it off the
shelf, who can design and make a compressor like this?

Spencers are too heavy: (http://www.spencerturbine.com/)
If Paxtons had more pressure, two might work:
(http://www.paxtonproducts.com/blowers.htm)
Vortons are close for two units, but no belt drives:
(http://www.vortron.com/products/blowers.html)