You may direct link sprites from this page if you have to, but if you're planning to use them on your site or something you should really just upload them yourself.
How it works: first, select the version whose sprites you want to get; then either press one of the three buttons to generate one, two or three random Pokémon, or enter up to three Pokémon into the text fields and then click the "Find sprites" button. Once the sprites have been generated, you can select a different version to immediately switch over to that version's sprites of the same Pokémon. This means that the radio buttons for the generations any of those Pokémon don't exist in will be disabled; if you just want to start over and get sprites for some different Pokémon in another version, use the "Reset form" button below the version selection.
Unown and Arceus can take a while to be generated, thanks to all their different forms. Just be patient.