You should only outsource your link building if
- You can afford relatively high fees for doing so
- You have no time on your hands
- You're in a competitive industry which genuinely needs powerful links
Firstly, you need to establish why you want links. Link building is a dangerous game.
Is it because your website is optimized yet your ranks aren't improving? Has a professional actually said, "there can be no more improvement on your website technically"? I would only ever consider link building when I'm out of ideas and in an incredibly competitive niche. Reason being, in most cases they simply aren't needed.
For example - A local hairdresser needs a good website, but they do not need links from the BBC. An insurance company trying to rank for "Insurance", would most certainly need a killer backlink profile. So the first decision you need to make is, do you really need links?
Now consider the least blackhat approaches possible. Google is very picky with which links they choose to actually classify as a vote. Many links are ignored, or actually, have a negative impact. For that reason, you need to decide whether you want to learn link building techniques or outsource to a genuine professional. The big issue here is many link building services will create quantity over quality. Finding a business which is genuinely good at what they can do can be challenging. And certainly not cheap. If you do decide to outsource, make sure to do your homework and avoid platforms like Fiverr.