You can buy Sculpey III (and other brands and lines of polymer clay) online from Sue Heaser's store in the UK:
http://www.polymerclaypit.co.uk
Or you can try locally at these places at least, I hear:
Hobbycraft
The Range
Compare prices, and note any sales or upcoming sales.
You can find even more sources for ordering polymer clay online on the Supply Sources page of my polymer clay "encyclopedia" site:
http://glassattic.com/polymer/supplysources.htm
....click on the category *Non-USA*
Be aware though that Sculpey III (and also original Sculpey and SuperSculpey --flesh) are weak clays after baking in any thin or projecting areas compared to all the other brands/lines. If your charms are fat and rounded and there's little stress on any holes you might make, there shouldn't be a problem with strength however. There will still be some problems with handling and creating details compared to other brands though, if that matters.
(If you're just looking for softness, Premo or even FimoSoft are still soft but will be much stronger after baking in any non-fat areas than the Sculpeys above).
Also, if you're interested, you can make all your own colors of polymer clay rather than counting on having to buy a bunch of different ones.
You can either mix the various colors together to get new colors, and/or you can mix colorants like oil paints and alcohol inks, etc., into white (or translucent, or any color) polymer clay --or mix the colorants before mixing them into the clay. Any of those techniques can result in literally thousands of new colors.
If you want loads more info about coloring clay and recipes for specific colors, etc., check out this page at my site as well:
http://glassattic.com/polymer/color.htm
(Also, for making connectors, or holes for connectors, in the clay charms, you might want to check out the lessons and tips on these pages too:
http://glassattic.com/polymer/pendants_cording.htm
...click on *Top Loops*
drilling holes all the way through the charms to hang them
http://glassattic.com/polymer/beads-holes.htm )
HTH,
Diane B.