Watch out with ebay though, as some of the Green Stuff on there could be very old and thus harder to use.
Ebob also sells Green Stuff:
http://www.ebobminiatures.com/products/tools.htm6" for 3 pounds, quite good value I think. He also sells some great tools and puppets.
Also there are other alternatives for putties. You got different sorts of milliput , you got ProCreate, and more. Each putty has it's advantages and disadvantages.
Generally speaking, milliput is cheaper, but not as fine as Green Stuff, cures faster and rock-hard, is pretty sticky and feels like working with clay.
Green Stuff is quite sticky, but when you press it, it usually 'bends' back a little, when curing it slightly expands, but is otherwise good quality.
ProCreate is probably a little more expensive than Green Stuff (unless you buy Green Stuff from GW of course), but has a very fine granularity, does not 'bend' back, is less sticky than Green Stuff and does not expand when curing. I find working with it a little easier than with Green Stuff. You can also make things more smooth (because of the finer granularity).
As you see, all putties have their own advantages, and therefore you can often find the right putty for the job. When sculpting a mini from scratch, I would say it's a good idea to start with Milliput as a base, use Green Stuff next for the larger parts and then add ProCreate for the details (like, head, hair, chain mail, belts etc).