In short, you need to trade Pokémon with a seller on one of the many sites that offer Shiny Pokémon. You can trade like you do with friends online.
In Pokémon Legends: Arceus, you need to talk to Simona in Jubilife Village to access online trades. If you subscribe to Nintendo Switch Online, you can trade with players from all over the world. However, before you can start trading Pokemon, you need to create a link code and share it with the person you want to trade with. You can Buy SWSH Pokemon here.
When purchasing a Pokémon from a site that offers Shiny, Alpha, or Shiny Alpha, you need to do all of the above through the payment system.
You can browse through numerous submenus and windows to find what you need. For most sites, you just need to find the shiny you want, add it to your cart (or equivalent site function), and enter your link code. After that, you need to choose the payment service you want and then actually pay some money.
Is this legal and safe?
There's nothing illegal or technically wrong with selling special Pokémon like this. The sale of digital goods has been around for a long time. This results in such procedures becoming more accessible and less dangerous. However, there are always risks when shopping online.
You can buy almost any Pokémon you want thanks to the convenience of online payment services that are popularized by the Internet. This is not limited to companies like Openbucks and Qiwi. Big names like Visa and Interac allow many sites to legally sell digital Pokemon.
As for which sites sell shiny Pokémon - there are many. While your first thought is that these sites are dangerous and just to rob you, that's not entirely true. You should always be careful when using such online services, although a large number of these sites are well designed and regularly updated.
The most popular ones like pkmbuy are apparently built by experienced web designers, which don't come cheap. But to provide services for Pokémon fans, the benefits and services are equal, and neither side will suffer. PKMBuy always provide useful guides and news for the majority of Pokemon players, you can check the guide and Buy shiny Pokemon. They will give a cheap and reasonable price according to the market price. There are Arceus Pokemon for Sale.