Morgan,

If using a fan as an impeller then the type of fan will have some effect.
The majority of small cooling fans are intended for largish volume and
lowish pressure and typically have 3 blades - you can see space through the
plane of the blade.

Next are semi axial fans where the blades have a degree of twist and the air
starts to be expelled at an angle to the axis of the shaft. Pressure is
higher and air volumes are reduced.
Some light still visible through plane of fan, but less.

Next are units where the air exits fully radially giving highest pressure
and lowest volume.
No light visible through plane of fan.



I imagine that one of the latter comes closest to matching the "impedance"
of your air supply. Using an electrical analog for the air supply, you
effectively have a "high voltage low current" air supply and you are trying
to drive a "low voltage high current" device. The radial fan comes closest
to matching the supply. Such a fan when used as an impeller would be driven
tangentially rather than having the air flow impinge on the flat of the
blades. I'm nit sure of your exact driving arrangement (in there somewhere I
know but too many posts in this thread now :-))

Best of all would (perhaps) be a "Roots blower" which is a positive
displacement device with two interleaving dumbell type rotors. This is a
very high pressure lower volume device.

The closest device in common use is probably a vacuum cleaner rotor. These
are driven by a series motor which is designed to rev up indefinitely until
the load matches the input power. Speeds are much faster than fans which are
synchronously driven ) 30000 rpm?. The "turbine" from a die grinder may also
be useful - and its fairly small. When used in a die grinder it takes far
too much air (and makes a laaarge amount of power).
It just may be that you could build.design a unit with magnetic bearings
inside your budget. Real units with such bearings are complex and take
extensive design effort but in your case the power rating is minimal and
efficiency unimportant. A magnetic bearing has, of course,  a superb design
life.




regards

                Russell McMahon