Buy veal or lamb kidneys; the others are not worth the time of modern cooks who go to work daily. Take the time and precaution to smell the kidneys you buy and never ever buy those wrapped in plastic. The smell of uric acid betrays an old kidney that no proper cooking procedure can make good tasting. Purchase kidneys from a butcher who understands that they are perishable and delicate and takes proper care of them.
Lamb and veal kidneys have no unpleasant smell or aftertaste once cooked, provided they have been cleaned properly and the cooking technique is quick and does not overcook the fragile tissues. Although stewed lamb kidneys as prepared in England for beef or lamb and kidney pie are quite edible, they do not in any way compare with kidneys that have been grilled or quickly sautéed.