Hi Jamie, i just asnwered your question at Thingiverse, but I’ll answer here again because I think this information could help someone in the future.
For buying stepper motors all you have to look is the voltage (12V
in this case), the rated current (I prefeer under 2A in order to control
it with cheap drivers), the torque (depends on each articulation, but
look at part list specs) and the size of the motor (in this project
there are all Nema 17 with different lengths, also provided in the part
list specs).
There are many websites along the net and local stores that sell this kind of motors, as them (except the geared motor) are frequently used on 3D printers.
If you want some links, these are the StepperOnline motors that fit with the specs listed here.
Articulation 1: 1x Nema 17 40 N.cm
Articulation 2 & 3: 3x Geared Nema 17 121.2 N.cm
Articulation 4&5&6: 3x Nema 17 26 N.cm
I think that would be nice to have a list with the online shops that have Thor components in order to ease the shopping.
Hope it helps!