“If it’s in stock, we have it” is a jocular store saying that has been printed on many images. “If it’s in stock we have it, and if we ain’t got it, you don’t need it” was printed in the Minneapolis (MN) Tribune on September 23, 1968.
 
“If we don’t have it, you don’t need it” is a popular related saying.
   
         
23 September 1968, Minneapolis (MN) Tribune, pg. 22, col. 5 photo caption:
‘IF IT’S IN STOCK WE HAVE IT, AND IF WE AIN’T GOT IT, YOU DON’T NEED IT’
So says a sign at Acme Electronics, which sells everything from TV tubes to potentiometers
   
17 June 1974, The Daily Chronicle (Centralia, WA), pg. 5, col. 5 ad:
OSCAR SAYS:
“It It’s in Stock
We Have It!”
(Cascade Enterprises, Inc.—ed.)
 
1 February 1976, Bellingham (WA) Herald, pg. 13, col. 1 ad:
TOBEY’S PHOTOFINISHING
IF IT’S IN STOCK, WE HAVE IT.
     
Google Books
1,001 logical laws, accurate axioms, profound principles, trusty truisms, homey homilies, colorful corollaries, quotable quotes, and rambunctious ruminations for all walks of life
By John Peers and Gordon Bennett
Garden City, NY: Doubleday
1979
Pg. 123:
Captain Risser’s Law: If it’s in stock, we have it.
 
22 May 1987, Washington (DC) Post, “For Washington’s Used-Book Shoppers: Something to Read” by Michael Dirda and David Streitfeld, Weekend sec., pg. 46, col. 1:
Fuller and Saunders’ slogan is, “If it’s in stock, we have it.”
     
Google Groups: rec.toys.action-figures
Spawn Repaint Question
Chet Jacques
10/8/96
(...)
Chet
McFarlane Toys
“If it’s in stock, we have it!”
 
8 February 1999, The Dispatch and Rock Island Argus (Moline, IL), pg. A10, col. 1:
BUSINESS QUOTE
“If it’s in stock, we have it.”
—business sign
     
Google Books
Small Time Operator:
How to Start Your Own Business, Keep Your Books, Pay Your Taxes and Stay Out of Trouble!
By Bernard Kamoroff
Laytonville, PA: Bell Springs Pub.
2004  
Pg. 98:
“If it’s in stock, we have it.” — sign in farm supply store, Ukiah, California
 
Vegas Seven
Can the W Hotel Revitalize the North End of the Strip?
ASK A NATIVE November 10, 2015 at 12:07pm By James P. Reza
Bonanza Gift Shop: Before and After
Last week’s question about The World’s Largest Gift Shop spurred additional comments and queries from readers.
(...)
Bonanza’s lease recently changed hands, and new operators are making changes that ruffle some locals, like removing the kitschy “If it’s in stock, we have it” marquee, and replacing the adult department (think The Hangover) with Kidzworld (think of the spectacular success of the MGM theme park …).
 
Trib Live (Pittsburgh, PA)
Paying ‘cash money’ for gas source of ‘mortified humiliation, shame
DAVE MCELHINNY | Wednesday, March 21, 2018, 12:51 p.m.
(...)
The prepay sign, which is known as tautology to word nerds like me, ranks right up there with “If it’s in stock, we have it,” “Unsolved mystery” or “Free gift.”
 
Twitter
Lion Circle Corp.
@LionCircleCorp
A little morning chuckle. 😊 “If it’s in stock, we have it!”
On a serious note, with Lion Circle you never have to worry about inventory issues because all of our products are always in stock! We have been producing quality, #MadeInTheUSA #promotionalproducts for over 100 years!
8:58 AM - 18 Jul 2018