They are quite hard to catch, only being a 1/8192 chance of meeting one. So you're usually going to run into Shinies in the grass. Or you can breed them using the Masuda Method.
For interactable Pokemon like Volcarona, you have to save in front of it, and talk to it, shut off and turn on the DS, talk to it, shut off and turn on the DS, talk to it, shut off and turn on the DS, talk to it, shut off and turn on the DS... until you get it Shiny. It's rare, so don't expect to get one right away or one at all.