This old classic brain teaser toy is now in Poundland. Just bought two of them for kids from Poundland in Winchester. Don't see any on their web-site, so not sure if it's nationwide. They had different colours in stock (red, green, black, blue). They are not rubik's official version and are called rubik's square. Absoplutely brilliant!
There are a lot of shapes to make you can find on the internet. For example - http://www.rubiks.com/World/~/media/Files/hint_snake.ashx