anywhere will sell you an outlet, the trouble with replacement parts is there is nothing standard in guttering and many are not compatible.
ie brent martin will not fit into everwhite, hunter not to polypipe ect at least not withut having to seal it with lead mate or something, luckly most guttering will have a makers name on it, usual places are the hopper or the downpipes.So first check it out for a makers name then just go get one of them.
if you find the name let me know and i will tell you were to get them.
would also like to add failing that just take the old one off and go around the merchants with it, try it into there lenghts of gutter till you got the right one then buy an outlet