GWs "greenstuff" is just Kneadatite from Polymeric Systems Inc. which you can get more of for less pretty much everywhere.

Having said that, if you are not used to it, it can be difficult to use - some people liken it to sculpting with chewing gum because it has a "memory" quality. It is pretty tough, even when cured it remains slightly pliable so will not suffer from brittleness like Milliput for example.

Have you tried "sculpting tabards" in youtube for a search. There are also miniature sculpting web-groups around that have forums where it is likely to have been a topic without you having to join up and ask again... sorry I can't think of the name of the one I was thinking off just now. If I remember, I'll post it.

Alternatives - Milliput as mentioned, Aves, and several other epoxies. If the miniatures are plastic, you won't be able to use polymer clays that need to be baked unless the parts are made separately and then attached.