You can get brown stuff (along with all your sculpting supplies) from Heresy (check out the link in the modelling resources sticky). I really like brown stuff, it cures to a nice hard finish with is easy to sand/file and is great for hard surfaces like armour and weapons. ProCreate is also very good - easier to use then greenstuff IMHO. Neither is a wonder material though, so it depends what problems you're having, but changing putty isn't likely to suddenly make you a much better sculptor.