They start roaming after you battle Silver(your rival), and fall through the trap door once you see them run away you can catch them. They are insanely rare, and run away once you encounter them so I would not recommend having Raikou on your team. I could recommend some other electrics though. If you do decide to have it on your team though try having Haunter(lv 40 or above) with mean look and hypnosis, and a Scyther(Also lvl 40 or above) with false swipe. Then use a max repel and roam around in some grass. Hope this Helps!