You can also stack two computer fans on top of each other to increase the amount of blow, not sure if more would have any more benefit than two.
Computer some fans can be powered from 9v to 24vdc with 58 CFM, 4350 RPM, 80 mm Fan.1
80 CFM 2
Of if you need some real grunt. 600 CFM @ 0.000-In SP, Voltage 115 AC, 60 Hz, Single Phase. 3
A bit harder to control as it would require a relay that you turn on and off to vary the speed.
Could hack up a fan speed detection system to have greater control over the fan speed.
Or there is the model aircraft fans and controllers a 4
(push force of maybe 3-5 kilos)