Most online retailers that cater to miniature wargamers/hobbyists will usually carry Kneaditite (Green Stuff).
The War Store carries it, and the manufacturer,
Polymeric Systems, has
this page that lists several other online distributors of their products.
The War Store has flat rate shipping, not sure about the other places. Generally you'll be paying $10-15 for the
GS (either six feet of the roll, or the same amount in separate pieces) plus shipping. As to the Tamiya sculpting epoxy putty, I found it no better than the blue/white epoxy putty
GW sold for some time, which is to say the Green Stuff is much, much better if only for it's color. Milliput is a different animal, I find it much better when used on vehicles, dreads and other things that need hard, sharp edges or need to be sanded smooth (though for gap filling I tend to use either
CA++ gap filling superglue or Squadron Green Putty).
Hope that helps! Enjoy!