Here you can find different sources of downloads for the the Vanilla World of Warcraft client, specifically patch 1.12.1. These clients are pre-installed so all you should need to do is extract them from the zip file and update your realmlist to the server of your choice.
Note that there have been many reports lately that Blizzard does file copyright strikes with ISPs for people using torrents to download their older clients. There are a couple of different options to help with this, we recommend using either a cloud based torrent client such as Bittorent or a VPN (Proton VPN is a solid choice, they have a decent free tier also).
Need to find a place to play? View our list of Vanilla WoW Private Servers.