@adambond,
I think there was a page on the Custom FAQ page dealing with something similar to this
I think mostly its called Aluminium Kneadite Epoxy putty, although Tamiya modelling putty which comes in a variety of colors is also quite good (and can be a cheaper option).
Some of the other putty/clay options out there require you to bake them, which is another reason why Greenstuff is pretty good.
You can get cheap clay or jewellery modelling tools from a dimestore or just use a wet tooth pick to make detail.
Also, be careful when sanding dried aluminium kneadite epoxy and always wear a dustmask and protective eyewear. It helps if you have a set of paint clothes for the task, maybe an apron and a ventilated area is a must.
It can be a little difficult at first sculpting a 100% scratch greenstuff mini, and I recommend an armature made of wire or a blank mini as a base to start with
Personally, im yet to master sculpting minis and have taken a break to study some art haha. But its all about practice and patience, and if youve had a pottery or fine-arts/ manual arts background then you'll find it comes quickly.
Apparently theres some new products coming onto the market soonish that dry translucent and a quick easymold material which would allow for small casting runs...
All the best and if you can take some before and after photos