Trade with other people who have heart gold or soul silver.
By cheating or trading with other people.
You can access Cerulean Cave in SoulSilver after you get all 16 gym badges (Johto and Kanto).
To get into Cerulean Cave you have to get all 16 badges in Kanto & Johto, beat Red in mt Silver, then talk to Prof. Oak
North of Cianwood City ( Kanto ), outside of Mt. Mortar in route 42 ( Kanto ), Vermillion City ( Johto ), Route 14 ( Johto ), then you find it for the last time in route 25 ( Johto ), where you will face it.
You can't- unless you trade
cheats
no only johto and kanto but you can get the 3 hoenn starters
Train them!
Once you defeat the all the Gym leaders from Johto and Kanto and the Elite Four go to Prof. Oak in Pallet Town and he will give you one of the Kanto starters.
defeat all the gym leaders in johto and kanto hope this helps
Get a friend to trade them to you.
You should be able too by trading Pokemon.
256 pokemon. But all together 493.
you get it once you have all the johto gym badges then go to Professor Elm and he will give it to you
keep seaching in kanto and johto everywhere and you"l find it
You need to complete the gyms and the elite four in johto first