If you have tried your hand at sourcing great places to place your content, you know it is not always easy. Using LinkedIn as a tool to find places to post is a great option.
Assuming you have a LinkedIn account, you should start joining groups that are in your niche. Once you have joined these groups, start looking around at the members of the group. Most likely those members have listed their website, and possibly their contact info.
Start contacting these members to ask if they accept guest posts, sponsored posts, etc... And don't forget to ask the members you connect with to share your info with fellow site owners/bloggers, as well. You will be surprised at how gaining an introduction from someone can open doors.