What you need is an assistant to do outreach on your (or your client's) behalf. They must have a system in place.
Here is a good
spreadsheet to find guest post options, and to track the backlinks gained.
This is a
document full of email templates to use to solicit guest post opportunities.
I suggest you shadow the blog, its owners, and any writers it has on your top social media accounts i.e. facebook, twitter, pinterest, youtube, linkedin, google+ etc. I suggest MP2 for retweeting,favoriting, liking their posts. You will also want to comment on their blogs before asking to guest post to add weight, value and recency to your email.
Obviously you will want to have content you can reference, to show you can add value, but with the resources I shared above, you should be able to hire an assistant at $3-$5 an hour to do this for you and get it all done, without having to worry about pbn link blogs etc.
Also, if you want to have some examples of content for niche blogs to brag about besides your own blog or media mentions from Press Releases, I suggest looking at this
list of paid links you can find from any forum, and apply to write content yourself (or client) on these sites, to build up a powerful portfolio.
