Is it too late to get Celebi and Arceus in Soul Silver?
The event for both Pokemon already passed, so you can only get them by trading now. Sorry.
Nintendo might announce another event for one of those Pokemon in Gen 6 though (Arceus is especially likely), so not all hope is lost. Nintendo are very unlikely to announce an event for HG/SS since they are old games now and the want to focus on the recent games of this Generation.
Unfortunately, both of them were limited time events that passed years ago. So, yes, it is too late.
Yes. By about 2 years. Celebi and Arceus are event only Pokemon. They cannot be legitimately obtained in-game.
They are both unobtainable. You may get them through AR, hacking, Pokesav and some other illegal stuff, or Nintendo may want to make an event. But right now, no event of any of them.
Source, plus my brain.