One of the most powerful normal-type attacks is Hyper Beam. While some Pokémon can learn this naturally, a TM is always a good way to make sure your team has such a powerful move. Fortunately, TM15, which contains Hyper Beam, is easy to find - you just need to have enough money to buy it.
The TM15 is sold at the Pietravelo Department Store. Veilstone is a relatively early game destination. You will go there to challenge the third gym leader, Maylene. The Department Store will be ready for use right from the start. It also stands out: just look for the large orange skyscraper with a green base. It will be northwest of the Pokémon Center.
Once inside, go up to the third floor and talk to the woman at the south counter on the left. It will sell tons of TMs and one of the more expensive ones will be the TM15, which contains Hyper Beam. TM15 will be sold for 7.500 PokéDollars. If you have the money, you can also make out with lots of TMs to overwhelm your rivals with.
Hyper Beam has a whopping BP of 150, but only PP of 5. Plus. Your Pokémon will need to rest for a turn after using such a powerful move, so be careful.