Hi,
on Milliput falling apart: try mixing in 30-50% green stuff. This will make the putty stay together better and after hardening the mix will be stronger. It will also be hard enough to file, sand, etc.
White stuff: kneadatite blue/yellow. Personally I feel the slight difference between green and white don\'t warrant the price difference (well, at least not when I bought some years ago).
Pro-Create: I wrote a review on it over here: http://groups.yahoo.com/group/1listSculpting/message/35252/ At this moment it\'s one of my main putties.
Brown stuff: if you tell me what part of the world you\'re in I may be able to suggest a store.
Hope this helps.
Bye, Ming-Hua