There are a few ways, but the first two ways have already expired.
The first two ways.
Get it from a wifi event in Pokemon diamond, pearl, platinum. Use the azure flute to summon arceus at spear pillar.
Get it from a real life event from game stop or Toys R Us.
The third way will never expire.
AR codes. (If I find more, I will add them. But if you don
t want any codes, I wont add any)
Press L+R while entering the canavil pokemarket.
I don`t remember if you get the azure flute, but if you do, here are the requirements
Have all badges
Capture dialga, palkia, or giratina
beat the Pokemon league
get the national dex
Once you get it, you may trade arceus to Pokemon heartgold or soulsilver.
The fourth way
Trade someone for an arceus