Where do you get the tube to fix power plant on heart gold?

You can't actually fix the power plant in Pokemon HeartGold. The power plant is not a broken object in the game, and there's no tube or any other item you need to fix it.

It's possible you're thinking of a different Pokemon game, or you might be misremembering something.

If you're having trouble with a specific part of the game, please provide more details about what you're trying to do and where you're stuck, and I can try to help!