Nowhere, of course, because it doesn’t exist.
Except in the mind of Beatrix Potter.
In the land of Green Ginger there is a town called Marmalade, which is exclusively inhabited by guinea-pigs.
They are of all colours and of two sorts–the common ordinary smooth-haired guinea-pigs who run errands and keep green grocers shops–and the kind that call themselves Abyssinian Cavies–who wear ringlets and walk upon their toes.
And the short-haired guinea-pigs admire and envy the curls of the long-haired guinea-pigs.
Both kinds of the guinea-pigs go to the Barber especially on Saturdays.
Hucksterism in the Land of Green Ginger in Beatrix Potter’s The Fairy Caravan
(Cotsen 21522)
The barber and his victim in Potter’s The Tale of Tuppenny illustrated by Marie Angel (Cotsen 11853)
If you want to find out about what happened when the bald guinea pig Tuppenny tried three treatments of Quintessence of Abyssinian Artichokes, the hair wash invented (and untruthfully promoted) by the barber, you’ll have to find a copy of The Tale of Tuppenny (1971) with illustrations by Marie Angel or The Fairy Caravan (1929), where Potter first published it.
The djinn. Noel Langley, The Land of the Green Ginger illustrated by Edward Ardizzone (author’s copy).
I don’t know if Noel Langley (1911-1980) learned about the Land of Green Ginger from Beatrix Potter’s Fairy Caravan. Perhaps not, because there are no guinea-pigs, just a djinn of the lamp, three suitors competing for the hand of lovely princess Silverbud, a feisty mouse, Omar Khayyam the tent maker, a flying carpet, and a dragon with a heliotrope tongue who likes his donkey with lettuce salad, tomatoes sliced thin. Everything necessary for a sequel to the story of Aladdin.
And who was Noel Langley? Born and educated in Durban, South Africa, he wrote and illustrated for children The Tale of the Land of Green Ginger (1937), which helped get him a seven-year contract with MGM. His Hollywood writing credits include The Wonderful Wizard of Oz (1939), Scrooge (1951), Ivanhoe (1952), and Snow White and The Three Stooges (1961).
Poster for the Wizard of Oz screenplay with Langley’s name at the head of the list of writers.
Langley also turned out plays, short stories and novels, but what he is remembered for besides the screenplay for The Wizard of Oz is his over-the-top pseudo-Oriental fantasy, The Land of Green Ginger, which he rewrote in 1966 and 1975, both accompanied by the illustrations of Edward Ardizzone.
The hero of The Land of Green Ginger, Prince Abu Ali, is not the likeliest of lads.
He was too amiable; too good-natured; too kindly; too honest, and too-fair-minded.
He was far too considerate of other people’s feelings.
He laughed too easily, and he was much too sympathetic.
He was deeply fond of both of his parents.
He was never lazy, impudent, or ill-mannered.
He could never raise his voice in foolish rage, or be a tattle-tale behind your back.
He was, in fact, quite hopeless.
Any sensible person would put their money on the other two suitors, Wicked Prince Tintac Ping Foo or Wicked Prince Rubdub Ben Thud of Arabia. Of the two, tall, skinny, mercenary Prince Tintac Ping Foo has the edge because of his way with words. Here is a conversation with his father, the Shah of Persia, in which it is revealed that Rubdub Ben Thud has stolen a march on Tintac Ping Foo.
“What? Rubdub Ben Thud?” shrilled the Wicked Prince in fiercest ire. “That balloon-faced butterball? Do you dare to tell me he has the silly sauce to pit himself against a paragon of loveable manly virtues like me?”
“I’m afraid so. Yes,” said the Shah of Persia.
“Oh har! Oh har! Oh, har!” scoffed Tintac Ping Foo scornfully. “I’d like to be there when they throw him out on his ear; but it’s far too far beneath my delicate dignity!”
“I quite agree,” agreed his father insincerely, “and I’d laugh as loudly as you my son; except that my spies inform me that Sulkpot Ben Nagnag looks with favor on his suit, and has invited him to lunch.”
The Wicked Prince Tintac Ping Foo went as purple in the face as a stick of jealous rhubarb, and shook his fists toward the sky.
“Then woe betide Rubdub Ben Thud!” he vowed vindictively. “He’ll rue the day he crossed my path! Ho there, Slaves! My camels! My retinue! My magic sword! My jellybeans! I leave at once for Samarkand.”
And what is more, gentle reader, he meant it and he did.
The author’s illustration of the wicked prince. Langley, The Tale of the Land of the Green Ginger (1937) (Cotsen 10198)
Prince Abu Ali and Silverbud united at last as illustrated by Ardizzone. The Land of Green Ginger (author’s copy)
But The Land of Green Ginger is a fractured fairy tale, so virtue will out, no matter which of the three versions you read. If you crave more of the rhodomontade quoted above, pass by the first edition and run, do not walk, to the 1975 version which is available in a handsome paperback from David R. Godine. Sometimes it is just too humid and hot to tackle the volumes like War and Peace, Infinite Jest, or 1Q84 on a summer reading list, whereas something light and frivolous like The Land of Green Ginger goes down like a scoop of coconut sorbet.
Perhaps, patient and forbearing reader, a small sigh has escaped your lips because this magical realm cannot be found on Google Maps.
The Land of Green Ginger which is a floating island.
Actually, if you go to the East Riding of Yorkshire and find the bottom of Whitefriargate in Hull upon Kingston, you can visit the Land of Green Ginger. 
The Land of Green Ginger in Hull.