How to catch Groudon and Kyogre?

I have finally made my way to the Elite 4 and I thought to myself maybe I should try to catch the big 3 before fighting the Elite 4. I saw someone say that I can only catch Groudon after I see the ending credits. I did check the weather station and there was no scientist for me to talk to regarding the weather patterns. I figure for now I’ll try an look to see if I can catch Rayquaza. Any help would be appreciated!