/r/reversejokes is a great little subreddit that I found. It's more or less completely inactive though, which is a shame.
Essentially, each thread is a punchline and the comments are the jokes.
Check it out!
Many of the jokes are contributions from our users. If you find anything offensive and against our policy please report it here with a link to the page. We will do everything to make this an enjoyable platform for everyone.