Generally oil paint is made with linseed oil. There are a few other oils used like safflower and walnut oil but generally linseed oil is the standard painters oil. Linseed oil is made from he seeds of the flax plant (pictured above) Flax has been cultivated for a long time, the ancient Egyptians grew it. Archaeologists have found died flax fibers over 30,000 years old. Most of the worlds flax is grown in Canada, but it also is grown in many other places including the northernmost parts of the Midwest.
Flaxseed oil or linseed oil is actually edible, however it is strong tasting and is usually used only as a health food supplement, as it contains high amounts of omega 3's.
The quality that makes linseed oil so good for making paint is that it is a drying oil. It polymerizes forming long chains of molecules. If you have ever had eggs dry on a counter top you have seen how tough polymer chains can be. Because they are rubbery and remain flexible they are close to indestructible.
Inh 1863 Fredrick Walton applied for a patent for a method of coating cloth backing with a thick layer of linseed oil and sawdust or cork. He called this product linoleum. That gives you an idea of how tough the stuff is when dry. An oil painting is basically a sheet of linoleum. Recent floor coverings are still called linoleum but are made of polyvinyl chloride (PVC). But there is plenty of the old stuff still out there and doing its job.
Oil painting are tough. There are virtually no 19th century men's ordinary clothes in existence. There are ceremonial outfits such as admirals uniforms and lots of women's garments. But almost no workman's clothes at all. They all got worn out. A pair of 19th century blue jeans found hanging on a hook deep in an abandoned mine in Colorado were sold at auction for 25,000 dollars. Oddly enough they were a pair of 34 by 33's, my size. The cloth from which those jeans were made came from the Amoskeag mills in Manchester, New Hampshire, still existing, though not as mills, I go there to draw figures once a week.
Unlike hundred year old pants, hundred year old oil paintings are very common, almost every antique shop has a few. Unless they are by a name artist or of a particularly attractive subject they may often be had cheaply. If people don't like a painting or it has gone out of style, they don't throw it away, they put it in the attic where it will wait patiently for generations to be rediscovered. Even if a painting is soiled or damaged, restorers refurbish paintings in dreadful shape, when they finish with them, they are as good as new, happens every day. For all artist's worry about permanence it is good to note that oil paintings are by nature pretty enduring.
Below is an example I found on e-bay today. It is 14 by 22 and has a minimum bid of 49.99. You can go bid on it by clicking here.
I could fill a truck with 19th century paintings in a week without leaving New England. They would still be worth less than the truck. Years ago I was invited into the home of a collector who haunted the auctions, buying 25.00 paintings. He wouldn't spend more. They were stacked against all the walls of his home, on the treads of his stairs, piled under his furniture and in the kitchen cabinets. After about half an hour I got bored with searching for a good one. I initially thought, with that many paintings there must be a treasure in there somewhere. I saw so much bad painting, I just couldn't go on. There was a lot of mass produced art and lots of grim amateur efforts. Just because a painting is old doesn't mean it is valuable.