Well Absol can be found in route 120 ( Ruby, Saphire and Esmerald)
In pokemon diamond and Pearl it can be found in route 213 but only as a Mass Outbreak.
In Platinium you find it in the snow part at Mt. Coronet.
You only have to walk in grass and if you're luckly it will appear very soon.
I would suggest use a great ball, but first low the HP and put it to sleep, into a Paralysis, or Frozen.