I have recently bought a used Leslie 760 amplifier, but unfortunately it came without the pedal used to change the speed of the rotator and which also acts as preamp. A used pedal and the necessary cables is priced at about €400 so I decided to build one myself.
Luckily, some nice people has posted scanned versions of the old manuals for Leslie amps and put them online: http://www.captain-foldback.com/Leslie_sub/leslie_manuals.htm.
The amplifier does not have a normal jack-plug for connection with an instrument but has instead a 9-pole plug. At first glance, this seems a bit confusing, but it is quite simple: Pole 1 is ground and pole 2 is sound input. The rotator is activated by grounding pole 6 (slow) or pole 7 (fast).
With this information it was easy to build everything I needed: I built a small box with room for two jack plugs to be put on the amp. The first plug is mono for instrument connection, and is attached to pole 1 and 2. The other plug is for stereo which is connected to pole 6 and 7 (and ground). At the other end of this stereo cable I attached a switch pedal with two buttons: One for switching between grounding the two poles of the stereo jack and another for switching the connection on/off. I got everything for about €25 on musik-ding.de.
I still needed a preamp and found one by the brand ‘Art’ at 4Sound.dk for €35, so for about 60€ I got everything I needed to run the amp.
You are more than welcome to write me at firstname.lastname@example.org if you have any questions.