It sounds like you received the Arceus through what's called a 'Nintendo Event', which are special free Pokemon (and sometimes items) given out by Nintendo and generally coinciding with a real-world event or promotion, for example a new game, movie or the opening of a store.
Currently there are no plans by Nintendo to release new Event Pokemon for any game older than the current Generation of games (6). As Heart Gold & Soul Silver are Generation 4 games, they will not receive any events.
What you may be able to do before it shuts down, is trade with other Players using the Global Trading Link/Service and request the Pokemon you are looking for. It is highly unlikely you will get what you want, unless you offer other rare or legendary Pokemon in return.
For reference, here is the Event-Pokemon list for Generation 4