They're rare, so you just have to find one by chance.
RARE ONES. Go see yourself!
first you go to the forbiden forest you get a big stick and force it though you head and you will find it there:)
Your rival's sister tells you about rare Pokemon appearances. Each day a rare Pokemon will swarm one route, if you go where she says, you'll find the Pokemon.
The Pokemon Mansion is home to some rare Pokemon. To find it in Pokemon Pearl version, go to route 212.
Go to Fuego Ironworks and use the pokeradar. Very rare
snow point temple. two rare Pokemon
you can use action replay or find them as you go on yourjourney
so you go walk around and find them in pokeballs on the ground.
Not really that rare... in Pokemon Diamond you can find Sunkerns on Route 204
Go into Pokemon mansion which is found on cinnibar island once inside wait until a wild Pokemon appears. Grimer is not that rare to find but you can find it.
the only way i know is to get Pokemon emerald and duplicate Pokemon holding rare candies(go to youtube to find out how)and migrate them to your game
you have to beat all hoenn gym leaders then you can go to the Pokemon league area and you can find legendarys there