i have used the universal chrome, flexible, "heat stove" material in the past to form a "hose", then go to seek one at the parts store.
although coat hanger or welding wire works as well, using the "heat stove" stuff allows a more complete visual aid to what the hose needs to be.
as to the size differences, most hoses today are so flexible, it "might" be able to be pushed onto a slightly larger radiator hose nipple.
however, you might get lucky at the parts store and find one of the correct size.
there are also "hose reducers" available if you find a hose that is larger than the radiator nipple.
don't forget to wind up a wire to fit inside the lower hose if it doesn't come with one. [most do not] this will prevent it from collapsing if the pump sucks really hard at a certain rpm.
1/16" or so stainless wire can be had from most good hardware stores, and can be wrapped around a broom handle or piece of pipe slightly smaller than the id of the hose.