“Buy land—they’re not making any more of it” is a statement that has been attributed to folk humorist Will Rogers (1879-1935) since the 1960s. The saying—like almost all American sayings—has also been attributed to Mark Twain (1835-1910). 
 
The version “ain’t nobody makin’ no more land” was used in Wyoming in 1909, so it is unlikely that Will Rogers originated the saying. Real estate companies popularized the Will Rogers authorship in real estate ads in the 1960s and 1970s.
 
 
Wikiquote: Will Rogers
William Penn Adair Rogers (4 November 1879 – 15 August 1935) was an American humorist and entertainer; known primarily as Will Rogers.
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Buy land. They ain’t making any more of the stuff.
. As quoted in Land in America: Its Value, Use, and Control (1981) by Peter M. Wolf, p. 6
. Unsourced variant: Buy land, they aren’t making any more of it.
   
ThinkExist.com
“Buy land, they’re not making it anymore”
- Mark Twain quotes (American Humorist, Writer and Lecturer. 1835-1910)
   
23 February 1909, Salt Lake Telegram (Salt Lake City, UT), “Farming Methods On Semi-Arid Lands Topic Of Big Convention At Cheyenne,” pg. 1:
The free prairie lands to the east of us have disappeared forever, and there “ain’t nobody makin’ no more land.” 
 
Google Books
Up-to-the-times Magazine
Published by Walla Walla Pub. Co., 1909
Item notes: v.4 (Nov.-Oct. 1909-10) 
Pg. 2731:
As a Wyoming cowboy expressed it: “The Lord ain’t makin’ no more land, and the feller that wants a farm better get it quick.”
 
5 September 1948, Charleston (WV) Daily Mail, pg. 8, col. 5:
They Are Not Making
Any More Land!
(Estill Realty Co, Realtors—ed.)
 
29 June 1952, Dallas (TX) Morning News, “Restaurateur Faithon Lucas Known as Excellent Farmer”  by Walter Robinson, part III, pg. 1:
Said Jolley: “He kept saying to me: ‘The land is precious; we are not making any more of it.’”
   
15 September 1952, Long Beach (CA) Independent, pg. B11, col. 3 classified ad:
NATURE AIN’T
makin’ no more land and it ain’ta gittin no cheaper.
 
20 September 1961, Valley Independent (Monessen, PA), pg. 20, col. 5:
In the windows of three real estate offices the past week I’ve noted cards bearing Will Rogers’ provocative quip: “Buy all the real estate you can—they’re not making it anymore.”
   
26 August 1962, New York (NY) Times, “Air Rights Offer New Supply Of ‘Land’ in Site-Pinched City” by Thomas W. Ennis, pg. 308:
“They ain’t makin’ no more land, hardly,” an aphorism attributed to Will Rogers, sums up a problem of urban real estate development in most large cities.
     
28 April 1963, Ada (OK) Evening News, “How Can land Get More Valuable”” by Cal Tinney, pg. 4, cols. 3-4:
It has been said, “They are not making any more land,” but they are.
 
14 May 1966, San Mateo (CA) Times, pg. 23, col. 3 ad:
DON’T YOU AGREE
With Will Rogers when talking about desirable land, who said “GRAB IT UP, FOR ONCE IT’S GONE, IT’S GONE…THEY’RE NOT MAKING ANY MORE!”
(Valley of the Moon Realty—ed.)
   
Time magazine
THE MERITS OF SPECULATION
Friday, Sep. 22, 1967
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For little money down and years to pay the balance, an Iowa farmer or Rhode Island schoolteacher can acquire without seeing it a small strip of Florida that is bound to quadruple in value—or so the salesmen hint, using a Will Rogers slogan, “Buy land, they’re not makin’ it any more.”
 
20 May 1968, Wisconsin State Journal (Madison, WI), section 2, pg. 11, col. 5 classified ad:
WILL ROGERS SAID “—BUY ALL THE LAND YOU CAN.—THEY AIN’T MAKIN’ ANYMORE”
   
21 May 1969, Cedar Rapids (Iowa) Gazette, pg. 15D, top banner:   
“Buy land..they’re not making any more of it!”
WILL ROGERS
 
28 June 1969, San Mateo (CA) Times, pg. 20, col. 2 classified ad:
Will Rogers Sez “Buy land;
They ain’t makin’ any more”
(Crotty & O’Halloran—ed.) 
   
Google Books
How to Buy Real Estate: Profits and Pitfalls
By Joseph Newman
Published by Books by U.S. News & World Report
1970
Pg. 49:
How to Buy Land
Will Rogers, the Oklahoma humorist, once offered his friends this bit of advice: “Buy land— they ain’t making any more of that stuff.”
 
Time magazine
The New American Land Rush
Monday, Oct. 01, 1973
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Citizens are taking seriously the advice of Humorist Will Rogers: “Buy land. They ain’t makin’any more of it.”