One type supply line that is commonly bought without people realizing the supply isn't stainless steel, is the supply lines sold at Home Depot.
Those are of a polymer braid, even though they look dead on stainless steel. Somehow I'll end up with those supply lines on the truck every now and then by accident, using them in a pinch when needed in a after hours scenario.
Watts supply lines have a very tight/dense stainless steel mesh pattern that completely covers the interior tubing inside the supply line. A very durable product and the different sizes (6", 9", 12", 16", 20", 30") work very well in hard to reach areas or in areas where it is difficult to move around fixed positions, being the supply line coming through the wall or the connection of the faucet.
We'll post some pictures of the stainless steel supply lines we use on a regular basis at Dunbar Plumbing. We will not use inferior products, even if the customer supplies us with the product.
If we do not feel comfortable with the product, we reserve the right to protect both of our interests in the matter of liability.