Link vendors - easy access to relevant sites and they can save you a lot of hassle. Of course depending on your budget number of links will differ. This is good for affilaite sites as most link vendors sell only link insertion or sponsored posts
Building a team- You need to have a SOPs in place and monitor them, but gives you flexibility in what approaches outreach wise you can use. Cheaper in long term and easier to scale
SEO/Linkbuilding Agency- Least hassle, flexibility with outreach approaches but higher costs
Individual freelancer- A middle ground between your own team and agency. Bit tricky to find good ones that will consistently perform.
Digital PR Agencys- They can create a awesome piece of content and promote it to the highest DR news sites but as you might imagine the cost is the highest.