Unfortunately there is no such thing as a standard 500 unit door. Although most manufacturers work to a standard width (300, 400, 450, 500 etc) you will find that there are various different heights available. For wall units these are usually around 700mm. B&Q will do a different height to Magnet for example. I have seen door heights vary from 651mm up to 904mm.
I would suggest measuring the precise height and width and search google with those measurements. Be aware that the door will be less than 500mm as 500mm is the width of the cabinet. I would imagine your 500mm door is actually 494mm wide.