How do you get Ditto on Pokemon Red?

Catch in routes 13, 14, 15, 23 and the unknown dungeon

Also, tips for getting answers quicker via google

Catch ditto with any type of ball on routes 13, 14, 15, 23 and in the unknown dungeon.

==============================================Tips for finding answers for games quicker:==============================================

Open google and search for "GAME_NAME walkthrough" where you have replaced GAME_NAME with your game's name, and you have removed the quote marks.

Click on the first (or if not, the most likely looking) link, and navigate until you find the actual walkthrough. Then press [CTRL]+[F] and type in what you were looking for (eg. "ditto"). Press find, and repeat until you are taken to a point in the walkthrough that has the information you want. If you can't find the information, try different links, or change you search terms slightly.