You likely already have it. Both GOG.com and Steam currently distribute Fallout 1 pre-patched to version 1.2. When you install the game from these stores, you are getting the official 1.2 executable, along with some basic DOSBox configuration.
If you are playing on a modern PC (Windows 10/11), install the following in order: fallout 1 1.2 patch
If you have the original CD:
Today, the 1.2 patch is considered the baseline for any legitimate playthrough. Most digital storefronts, including Steam and GOG, come pre-packaged with this version of the game. However, for those running original CD-ROM copies, hunting down the 1.2 executable is a mandatory first step. It represents the bridge between a flawed masterpiece and the polished cult classic we celebrate decades later. You likely already have it