There are many ways to find do-follow links as answered above but also these things should be kept in mind-
Manual research on various sites with do-follow links - Manual research on sites on which you are posting your content is so necessary because some sites only allow relevant content for do-follow links.
Creating database of quality based sites - Manual research will bring you so many sites data with do-follow links, Setting a database is also important for such tasks.
Things to avoid-
Don't use irrelevant content - Don't post irrelevant content on sites. Relevant content will lead to extra benefits.
Don't add yourself to a trash can - If the website's full of crap - meaning it has lots of outbound links and it's content ain't too shabby, question it. Investigate further what the site's about. Look at all the outbound links it points to.