You would do better to stay away from the Chinese motors altogether if you can afford to. They are a low price because they are cheaply made. That presents itself in various ways, they are generally still quite reliable but their operating parameters are variable in that the steps can be uneven sizes and the current they will handle can vary within a motor for each of the coils along with the torque they produce per winding. Microstepping is where they really fail, the steps are hugely varied.

As steppers are very reliable and rarely fail unless they are dismantled buy second hand. The cheapest way is to buy bits of a machine with them on usually.

Stuff like this though this isn't a stepper.
http://www.ebay.co.uk/itm/2719217173...%3AMEBIDX%3AIT