This post comes from having an underutilized vertical monitor. The two things I make use of the most in this post conveniently happen to look great vertically. Do not adjust your monitor. You’ve probably had a little play on using Twitter as a method of first contact before, but you may not have used it as a way to source potential placements. Twitter is excellent for both of these.
So who’s most likely going to be wanting guest posts? A good place to start would be the people asking for guest posts and the sites already publishing them. Websites usually tweet every post they make and often differentiate whether the post was written by one of their team or an outsider. They do this either in the tweet or in the URL structure.