Well, you could try starting at the entrance and drawing a crude map on paper to help you figure out where you are...explore every which way and you should find Shadow Pokémon trainers again. If they don't want to fight you again, don't despair...just continue with the game, and after you've beaten the final boss and cleared all the plot points, there's a place where you'll be able to randomly encounter any Shadow Pokémon Trainer again...with one exception. The very last Shadow Pokémon trainer, the one with the Togetic, can only be encountered once, so even though it's not a difficult catch by any means, you might want to have the Master Ball handy if you're feeling paranoid.