Wailmer Help!

Basically, I;m starting Pokemon Sapphire again. And this time I thought I would make it through to the Elite Four with one fixed party without catching any other Pokemon (escluding the Safari Zone, where I will catch some reserves). I want my Party to look like this in the end:
-Blaziken,
-Mightyena,
-Swellow,
-Wailord,
-Tropius,
-Pikachu (Raichu?).
Here's the thing: I am now entering the Dewford Gym with my Combusken at Lv. 17, Poochyena at Lv. 15 and Taillow at Lv. 15. Where is the nearest spot where I can catch Wailmer and with which rod? Please, help!