Guest Postings for a Digital Marketing Agency?

Hey! I was wondering what could some good sites be for guest postings for a Marketing Agency? I mean websites related to that would be providing services of its own. Therefore, they won't provide a backlink to their competitor.

You can broaden the niche to business, investing and craft articles like --->

1) How to get more out of your investment by using the right marketing agency?
2) How a right marketing agency can increase your business?
etc.