I can recommend British Organ Blowers (B.O.B.) of Coleman Street,
Derby, UK.  They are a very old company, still trading but making
fans mostly for ventilation plants; however, as their name implies,
they started off making blowers for organs.

They will provide the correct size for your pressure and flow
requirements.  Their blowers may not be the cheapest but quality
doesn't come cheap.  I use one of their blowers in my street organ and
it has performed faultlessly for twenty years.  B.O.B. will have the
expertise to specify the correct blower for your organ, given the
pressure and size.

To answer Robbie's question, most new organs on blowers, and old organs
fitted with blowers in place of feeders, retain their reservoir and
spill valve.  This ensures that over-pressure and -flow supplied can
be easily disposed of and the pressure supplied to the pipes will
always be correct.  I have seen one new organ where the blower fed the
chest directly and this worked perfectly.

The problem with this design is that the blower supply pressure must
be matched exactly with the pipe pressure and the blower maximum flow
must accommodate the maximum demand for the pipe chest.  Using the
traditional sprung reservoir permits the fan to be under-sized in terms
of flow so heavy pipe demands are covered by the reservoir, and the
reservoir is then replenished during later more lightly winded musical
passages.  One must take care, however, not to go too small on the
blower, otherwise a very long highly winded passage will run out of
puff!

Another possibility, which I have never seen on a fair or street
organ, but which is used predominantly in church organs, is to feed
the reservoir through a gate valve which is mechanically linked to the
height of the reservoir.  A full reservoir, therefore, cuts off its
input.  This is only possible when using a centrifugal blower as such
a blower will still run when stalled, albeit off load and therefore
slightly faster.

The traditional fair organ feeders are constant displacement pumps and
if stalled will most probably explode.  I presume it is the familiarity
with the traditional triangular rise reservoir with spill valve that
retains its use with blowers in fair organs.  B.O.B.'s address is below.

Regards from the UK,  Nicholas Simons.

B.O.B. Stevenson Limited
5 Coleman St., Alvaston,
Derby, DE24 8NL
United Kingdom
tel.: +44(0)01332 574112
http://www.bobstevenson.co.uk/ 
http://www.bobstevenson.co.uk/british_organ_blowers.htm 
http://www.bobstevenson.co.uk/organ_blowers.htm 
http://www.bobstevenson.co.uk/organ_blower_fans.pdf