Manufacturers
I am aware of three companies manufacturing compatible motors. All appear to produce a good-quality product.
Switec 
Switec was originally the technology arm of the Swiss watch company Swatch. The stepper business was sold off to Singapore-based Juken Technology in 2009. Switec motors are white, and have part numbers starting with “X”.
VID 
Hong Kong based VID also manufacture a range
of similar motors. Their motors are black, and have a black model number starting with “vid”.
MCR 
Chinese company MCR Motor manufacture a range of compatible motors.
Their motors are white. with part numbers starting with “MR”.
X25 Product Numbers
Each manufacturer produces a number of variations of the motor.
| Switec | VID | MCR | Contacts | Mounting Pegs | Internal Stop | Shaft |
|---|---|---|---|---|---|---|
| X25.156 | rear | no | no | standard | ||
| X25.166 | rear | no | yes | standard | ||
| X25.158 | rear | rear | no | standard | ||
| X25.168 | VID29-02 | MR1108 | rear | rear | yes | standard |
| X25.278 | rear | rear | no | long (+2mm) | ||
| X25.288 | VID29-04 | rear | rear | yes | long (+2mm) | |
| X25.559 | front | front | no | standard | ||
| X25.569 | VID29-05 | MR1107 | front | front | yes | standard |
| X25.579 | front | front | no | long (+2mm) | ||
| X25.589 | VID29-07 | front | front | yes | long (+2mm) | |
| X25.679 | front | front | no | long (+2mm) | ||
| X25.689 | front | front | yes | long (+2mm) |
Other Switec Series
X23 Series
The X23 motors have an a hollow metal shaft encasing a transparent light guide to make it possible to have an illuminated pointer.
X26 Series
The X26 motors are dual-shaft analog clock motors. The model numbers are X26.101, X26.103, X26.123, X26.504. See the the “Switec Series Buyers Guide” in the Resources for more details.
X27 Series
The X27 motors are a new line replacing the X25 series. The X27 part numbers mirror the X25 series.
X12 Motor Drivers
The X12 part numbers refer to a series of driver chips developed by Switec. I have not been able to locate a supplier of these chips since microcomponents.ch shut down.
Suppliers
I have not found any well-known online store supplying these parts. They are cheap and plentiful on ebay. However it can be difficult to determine exactly what you are getting; these motors are generally sold as Switec X25 replacements, without clearly specifying the manufacturer.