The most common places that they’re available are the Super powerup event and the Super Challenge Battle. However, recently the devs have started putting Omegamids in new places to help beginners out a little more, so there’s really no telling when the next good opportunity will arrive. Just keep watching for it and try to be ready.
Also, just a heads up, it’s generally not good forum etiquette to reply to a very old thread like this one, so in the future please pay attention to the dates of the thread you’re posting in. If you can’t find one, then just go ahead and start your own in the appropriate section. In the meantime I’m going to lock this thread.