The smallest kind of hairdryers contain an electric motor and a fan unit which are ideal for recreating a jet engine suitable for a 1/33 scale paper model, which will run on 9V. Below a few photos to demonstrate.
1) At the flea market, look for the smallest (least diameter) hairdryers, like the one in the image. This one cost me 2 euros.
2) A certain amount of judicious violence lays bare the treasure at the middle, the fan unit (plus, if you're lucky, a small usable switch). Cut off the heating spiral and all electronics.
3) And this is what you're left with - one small electric motor with a very good fan attached to it (the fan is the main reason to go to this trouble), plus a usable switch. The largest diameter of this unit is 27,5 mm.
4) The beauty of it all is that although the hairdryer as such is plugged into the 220V mains, the electric motor detached from the heating spiral runs like a dream on a 9V battery (preferably rechargeable, for mounting inside the model).
The high-pitched wining sound and powerful airstream are most realistic!