There are zillions of people selling them, they just like to put their own number on so you have go to them for replacements.

Not 100% sure but I think these are the manufacturers:

Stepper motor|Stepping gearmotor wholesale|Stepping motor supplier

If you are thinking about buying one of their kits there are a few posts on cnczone moaning about how under powered the PSU is, might want to take that into consideration.