The best place to download the Pixelmon Mod for Minecraft is through pixelmonmod.com. It offers multiple versions of the mod for download through a safe hosting site.
From the main page of Pixelmon Mod, click on the "Downloads" tab. This opens up the Pixelmon Core Downloads. On this page, select the version to download. Pixelmon has a list of recommended downloads that are versions of the game which have been tested and debugged.
Players also have the option of downloading the Beta version and a Windows based Pixelmon Mod launcher. The launcher makes it easier to launch this mod in Minecraft for those who are not as familiar with installing game mods.