Senefelder was the first to try and make imitation stones as early as 1814…. white lead, lime, and caseine…His invention came to be known as paper stone…Knecht, Senefelder’s nephew, made a more resistant imitation stone using the following ingredients and proportions:
chalk………………3 parts
silver white………1 part
linseed oil…………1 part
lime………………..1 part
iron oxyde………..1/8 of a part
It was said that this “stone” could handle some six hundred impressions.
—A technical dictionary of printmaking, André Béguin.
There is a worry I have heard expressed in the litho studios here in China:
If all the litho stones in the world come from one quarry somewhere in Germany and that quarry is closed now, then every time a stone breaks the lithographs get smaller and smaller. Soon there will be no stones. One artist asked, “well couldn’t we discover stone in China? Has anyone ever looked?”
Apparently there have been multiple sources of litho stone, but not all sources were of equal quality. Another issue is that stone lithography is no longer commercial and therefore re-opening quarries or exploring new sources is not economically feasible.
litho quarries in Europe:
Towards 1920 the Solenhofen quarry began to show signs of depletion but the crisis of lithography saved it from being exhausted.
—A technical dictionary of printmaking, André Béguin.
For many years, the Solnhofen deposits were the only source of lithographic limestone. French lithographic limestone from quarries near Montdardier, about 6 km south of le Vigan, Gard was exhibited at the Great Exhibition of 1851…The largest lithographic printing stone ever quarried came from Le Vigan, 230x150cm (90x59 in).
Le Vigan Region, Gard, Montdardier. A quarry for lithographic stones d’Avèze. This image was taken around 1910.
The American Lithographic Stone Company was organized in Louisville, Kentucky in late 1868. It initially focused its operation on quarries in Overton County, Tennessee,[13] but shortly before 1900, it opened a quarry at Brandenburg, Kentucky. This quarry was the only commercial source of lithographic stone in the United States at the turn of the 20th century. Unlike the Solnhofen stone, Kentucky lithographic limestone was slightly dolomitic, and it was judged to be competitive with Solnhofen stone for some purposes, but not for the highest quality work.[14][15]
—Lithographic Limestone, Wikipedia
One quarry in the US was found to have stone of similar quality to the Solnhofen stone:

At the turn of the 20th century, a town in north-central Iowa was founded because of this interest in high-quality lithographic stone…The quarries operated for only a short period, however, and the town failed to prosper as metal engraving replaced lithographic stone in providing good quality printing at lower cost.In 1968, a representative of Tamarind Workshop visited Iowa to evaluate the potential of using stone from Lithograph City. Preliminary results indicated that its quality compared very well with Solnhofen stone…In the course of their studies, however, it was discovered that white onyx could be used as a substitute. —Bill J. Bunkee (Adapted from Iowa Geology 1991, No. 16, Iowa Department of Natural Resources)