Of Australian-English origin, the humorous phrase as scarce as rocking-horse manure, and its variants, mean very scarce.
—Cf. also the phrase as scarce as pork chops in a Jewish boarding house.

The noun rocking horse denotes a toy horse mounted on rockers or springs for a child to sit on and rock to and fro.
—Cf. also origin of ‘dada’ (‘hobby’) and ‘Dada’ (artistic movement).

The earliest occurrences of the phrase as scarce as rocking-horse manure and variants that I have found are as follows, in chronological order:

1-: From Market for Stable Manure, published in The Dampier Herald (Kununoppin, Western Australia) of Thursday 12^th August 1937:

A sideline which wheatgrowers have overlooked is the export of stable manure to vegetable growers in the metropolitan areas (states the secretary of the W.A. Co-operative Producers’ Association, Mr. B. V. Brooks). His association was formed by metropolitan vegetable growers with the object of extending vegetable markets, both locally and overseas. Five years of cropping with vegetables exhausts the land, Mr. Brooks said, and unless the land can be built up, the growers must move to new country. Stable manure was the best soil builder, but the growth of motor transport in the city had made stable manure almost as rare as the fabulous rocking horse manure. Vegetable growers had been forced to go far afield for supplies. If wheatgrowers could arrange to deliver stable manure in truck lots to Perth, vegetable growers would guarantee a payable price.

2-: From The Vegetative Eye (Melbourne and Adelaide: Reed & Harris, 1943), by the Australian author Max Harris (Maxwell Henley Harris – 1921-1995):

NEW URGENCIES WILL ALWAYS EMERGE TO MAKE THOSE RARE MOMENTS OF CLARITY AND CREATIVE RETROSPECT AS RARE AS ROCKING HORSE MANURE.

3-: From Sojourn in Tobruk (Sydney: Ure Smith, 1944), by Geoffrey Harry Fearnside (1917-1978)—as quoted by Gerald Alfred Wilkes (1927-2020) in A Dictionary of Australian Colloquialisms (Sydney University Press in association with Oxford University Press Australia, 1990):

Australian cigarettes were as rare as rocking-horse manure hereabouts.

4-: From Notes from the City, published in The Wingham Chronicle and Manning River Observer (Wingham, New South Wales) of Tuesday 10^th July 1945:

To-day this column strolled into a bureaucrat’s hive—there are 12 or 13 stories in it, filled to capacity with the merry boys—and made an inquiry on behalf of a rural friend and colleague as to the possibility of getting a spot of help. “What?” exclaimed the pleasant nabob in charge of that particular division. “Do you know that instead of having one of these birds waiting in the cage, there are a dozen applications coming to hand for them every week or so. Why, they’re as scarce as rocking-horse manure!” “Well, put me down for a dozen bags of that instead,” said this department as it made for the swing doors.

5-: From Column 8, by ‘Granny’, published in The Sydney Morning Herald (Sydney, New South Wales) of Wednesday 28^th October 1953:

Many North Shore gardeners have been victims of what they call the Rocking Horse Manure Racket.
Knowing it is difficult for them to get manure, itinerant sellers have been calling at homes with bags of “special” manure—treated sawdust.

The phrase as scarce as rocking-horse manure and variants have come to be also used in American English and British English—for example:

– In Paulsen ¹ Discusses His New TV Image, by Jerry Shnay, published in the Chicago Tribune (Chicago, Illinois) of Thursday 25^th December 1969:

Paulsen is more than just a family man. He has loyal ties to both Smothers brothers ², especially Tom, and has stated opinions as to the outcome of their lawsuit against CBS. He also realizes the value of friends, and this, in Hollywood, is about as rare as rocking horse manure.

¹ Patrick Layton Paulsen (1927-1997) was a U.S. comedian and satirist.
² This refers to the Smothers Brothers, i.e., Thomas (born 1937) and Richard (born 1938), U.S. singers, musicians and comedians.

– In this advertisement, published in the Evening Standard (London, England) of Tuesday 29^th October 1974:

RARER THAN ROCKING HORSE MANURE
BLACK BEAUTY 230.6
Mercedes Limousine

8-seater, 4000 miles. Tan upholstery. Cost £6300 new, would accept £5500, part exchange considered. H.P. arranged.

– In At the movies, by Tom Sullivan, published in The Herald-News (Passaic, New Jersey) of Sunday 20^th November 1977:

They keep saying around Hollywood that if anyone writes a good screenplay it will get made into a film because good writing is harder to find that rocking horse manure, right?…Right! And David J. Kinghorn, a teacher at Sonoma County juvenile hall in Santa Rosa, is living proof…His “Specter” has been purchased by Universal and will be filmed by Carl Foreman. It’s his first stab at screenplay writing but said to be awfully good.

In the phrase, manure has occasionally been replaced with the noun shit. For example, the following is from The rank outsiders, by Mathew Engel, published in The Guardian (London and Manchester, England) of Saturday 5^th March 1988—the author has joined the ranks of Anzio Platoon “to see how British youth is miraculously transformed into trained fighting men within 19 weeks”:

Next they meet Regimental Quarter Master Sergeant Farrow and if they didn’t know they were in the army before, the recruits know it now. RQMS Farrow is about to issue their kit. He is issuing them with Army property valued at £453.30p (the exact amount varies slightly at different points of the lecture but they get his drift) and the recruits are going to take care of it.
All of it: Boots, combat high- . . . Helmet, combat . . . Gloves, combat (“These are like rocking horse shit. You can’t get ’em. We only get four pairs every three months to exchange. You will not get a replacement pair. So don’t lose your gloves, combat.”)