I have an nearly new (only hooked it up once to see how it works) Night Fly Servo Slow Down unit (NF-SD1). It is a small and light weight unit that comes with instructions and has the following features:
Operation voltage: 4.5 volts to 5.3 volts Slow down time: variable rate between 0.7 seconds to 14.7 seconds.
Perfect for flaps.
Asking $20 shipped within the US. Paypal accepted at email@example.com.