- Masterball x4 (one left over to clone more), to catch them easily and without fail.
- Cloning cheat, to get more master ball.
- a lot of Super or Max Repel, to keep away Pokemon that will interfere, if you find a Pokemon before them they might run away and you will have to track them down again.
WARNING: You must have room in your box to store them!
Now that you have that stuff we can try and track them down. I got lucky and found Entei by accident while coming down the path below Black Thorn City. After I found Entei and got him to a place I could get him, I found Suicune instead, I caught him but my box got full afterwards and I had to abandon my search for Entei. Always have room when hunting them.
The most common place I've seen them is the route before the Lake of Rage, Suicune especially. Entei and Raikou and pretty commonly seen right before the house leading to and in route 29 (The route near your house where you can find Lv 2 stuff), where you can also find Jigglypuff. I've never actually caught Raikou because this masterball plan is so new and I haven't tracked him down yet. Raikou is the hardest one I've tried to find and capture, good luck with him.
This plan isn't essential to catching the Legendary Dogs, but it is easier. Also, you should note they will be on Lv 40 when you see them and any damage you do is permanent until captured. There is only one in each game and if you kill them there will be no more. If you plan to weaken them and then catch them you will need a fast Pokemon like Golbat with Mean Look to trap them in battle and weaken them. I suggest saving before the hunt just in case you kill one. I hope this helps.