Available in-world at Spiky-Haired Dragon, Blorkus secondlife://Blorkus/251/176/110, in boxes of 4 or as single colours.
Box of four "Naturals": Blue, Green, Brown, Grey -- 50L
Box of four "Unnaturals": Red, Gold, Purple, Turquoise -- 50L
Single colours -- 15L each
And the pink ones -- free!
Boxes and the free ones also available at SLX:
http://www.slexchange.com/modules.php?name=Marketplace&file=item&ItemID=681885
http://www.slexchange.com/modules.php?name=Marketplace&file=item&ItemID=681888
http://www.slexchange.com/modules.php?name=Marketplace&file=item&ItemID=681890
and Onrez:
http://shop.onrez.com/item/615920
http://shop.onrez.com/item/615922
http://shop.onrez.com/item/615921