You can capture a wailord in route 230. If you can not find it you better capture a wailmer and evolve it.
completely by chance
route 223 or route 230
On the route below the Pokemon League. Rare though.
on heartgold and soulsilver level up wailmer to level 40. on diamond pearl and platinum go to route 223 and use the super rod and then you will have a wailord.
it can breed with seaking, gyarados, lanturn,qwilfish,octillery,sharpedo,wailord,whiscash,relicanth,luvdisc,lumineon, and all of their previous forms
If you fish using a super rod you can catch a wailmer, then just level it up and Voila Wailord
You can go fish at route 223 and route 230 with a super rod and eventually you'll find one
You need relicanth and wailord to catch the regis in sapphire/ruby/emerald. You cannot catch the three regis in platinum unless you have the event Regigigas (this event has now ended). If you have the special regigigas in your party, three new caves will open up in Sinnoh and you will be able to catch the three regis.
You can catch level 40-55 Wailords with the super rod at Route 223 and Route 230, but the chance of a landing Wailord on your super rod is very rare, at 5%. Level 30-40 Wailmers are much more common, at 30%. To save time, catch a Wailmer with a super rod and train it to level 40 and it will evolve into a Wailord.
Wailord
Wailord does not currently evolve into any pokemon, but it does have the pre-evolution wailmer.
you can get dragonite,lucario,eevee,gardevoir,wailord,dialga palkia,giritina ,arceus,etc.