Kidneys are a very popular part of the pig in ethnic cuisines (except, of course, those that are forbidden pig). The photo specimens were 5 inches long, 2-1/2 inches across and 1 inch thick, weighing 5 ounces each. If you must substitute, use lamb kidneys.
More on Cuts of Pork.
Buying: These can generally be found in ethnic markets (except those serving primarily Jewish or Muslim communities, of course). They are particularly common in markets serving Chinese, Southeast Asian or Southeast European communities. They will all have a slash cut from one side through to the center, for drainage or inspection, I'm not sure.
Prep: Most often you will slice them in half horizontally (as the center one in the photo above) to expose the fat and membranes for removal. A razor sharp boning knife is good for cutting away fat, membranes and plumbing. Cut away all white, leaving all dark. If the kidneys are to be sliced, I've found it easier to cut the white stuff away after slicing.
Some Asian markets sell them "cleaned". Actually they slice off both sides and discard the center slice.
While others may use more complex procedures, I find that, after removing the fat and slicing or cutting the kidneys to the size needed by the recipe, a simple rinse in several changes of cold water does fine for cleaning them up.
Cooking: As with other innards and the kidneys of other beasts, these are best cooked either very quickly or for a long time. They will be tough in between.