Answer: look around for Pidgeys, Rattatas, Gulpins, Sentrets, etc. Catch enough of them and eventually one will turn into a ditto.
Not an answer: I wonder if trading for a Ditto would satisfy some of those quests. Suspect not, because it would make for an easy hack, just save all your dittos, and when someone else needs one, trade one away; when you need one, trade for one, etc.
