Welcome to the Q&A Forum

You can use Twitter + FollowerWonk or Google (the search engine) + custom queries. You can type things like these on Google to find potential guest posting opportunities: "keyword" + "guest post" "keyword" + "write for us" "keyword" + "submit your post" etc. You might be interested: http://findmyblogway.com/finding-guest-post-opportunities/ (discusses about some unique ways)