BigPlanes selected the retract systems from Electron, CYMODEL, SEAGULL, SLR, TOPRC and ESM. These very nice and robust retract systems are available in many flavours.
Except for electric retract system we also have pneumatic (air) systems. There are often more options available for a single model both in systems as well as struts and wheels.
There are several electric versions available: Electron, SLR, CYMODEL, and ESM.
It is also possible to create your own retract system by selecting a strut from e.g. Robart or ESM and a related retract mechanism.