for the water pump if youre replacing the whole thing as a complete unit then you need the thick cardboard like flat gasket to mount it to the back of the engine block.
if youre only replacing the "outer" impellor/pulley part of the pump this requires a rubber sealing ring approx 3 inches in diameter between it the mating surface on the pump housing. try a motor factors for the right one.
I wouldnt risk a used water pump myself as its such a pain to renew them unless you know its barely used and will be reliable. Brand new ones are only £35 (light pump) and come with the right gasket and usually a 2 year warranty.