Kits are available from Talking Electronics.

Sometimes the servo may be reversed by rotating the whole joy-stick assembly on the transmitters. Reversing the wires on the joy-stick pot itself is another way, but this is not always possible either. Only a fraction of the pot's travel is used, and it is not always the central portion of the pot's resistive track.
Sometimes, no matter how you look at it, there is no easy cure. Replacing the servo with a reverse operating one, if available, is an expensive way to cure the problem. A cheaper alternative is to install a simple servo reversing circuit such as this one.
Not many components are required to do the trick, and they can be mounted on a tiny printed circuit board that can easily be accommodated in a model. So how does it work?
Each servo's control signal is sent from the transmitter as a 1.5 to 2.5 millisecond pulse. The pulse is repeated roughly every 20 milliseconds.
What we want to achieve is to mirror the pulse variation. When we have a 1.5mS signal we want a 2.5mS signal and vice versa. The easiest way to do this is to subtract the incoming pulse from a 4mS pulse. 4 minus 2.5 equals 1.5! And the best place to do this to the signal is right between the receiver and servo.

HOW THE CIRCUIT WORKS
First, it should be noted that this project only works with positive pulse digital proportional radio control systems. Most, if not all of the common brands use this system.
The signal from the receiver is buffered by the first two inverters and sent to the output NOR gate which consists of two signal diodes, a 100k resistor, a 1n capacitor and a Schmitt inverter. It also fires the monostable built around the BC547. The 10n capacitor passes a narrow pulse to the base of the BC547 every time there is a pulse at the input. This turns the transistor on briefly, charging the 22n capacitor. This capacitor then discharges through the 47k resistor and 100k trim-pot. This, when buffered by the two Schmitt inverters in series, provides a LOW level pulse of 4mS duration. This pulse is also fed to the output NOR gate. The resultant pulse from the output of the NOR gate is fed to the servo. The 1nF capacitor that is part of this NOR gate is to compensate for a slight delay between the falling edge of the 4ms pulse and the rising edge of the buffered receiver output pulse.

CONSTRUCTION
All components are mounted on a small single sided printed circuit board. The resistors and diodes have been stood on end to conserve space. Take special care with the orientation of the diodes. It is very easy to accidentally reverse them when they are stood on end.

The output to the servo is from a three pin header on the PCB. It is not polarised, so take particular note of the markings on the overlay when you are connecting the servo. The white or other colored wire should go to the pin marked "S". The red wire should go to the centre pin and the black or brown wire should go to the pin marked "-". This is the most common servo wiring order. The Kraft servos I own use a different order. The colors of the wires still clearly show which pin is for which job, so in such a case, wire the 3 pin header off-board on a lead.
The wiring order for the input connector to the servo reverser is the same. The only difference is that the pads for connection are in different locations around the PCB. They are clearly marked.

SETTING UP
Connect the servo directly to the receiver and centre it using the joy-stick and trim. Note the position. Now connect the servo reverser in line with the servo and switch the receiver on again. Adjust the trim-pot on the servo reverser board until the servo is centered at the same position it was before. Be quick with the adjustment because it is possible that the servo may initially try to move past the end of its travel. It is possible to use this trim-pot as a local trim control if you desire. Once again, take care not to let the servo try to move past the end of its travel.