You'll be able to contract out the designers directly who made the original icon set. No worries about quality because it's the same person who did the original. They'll match perfectly, and you'll be able to work with them to make sure you get exactly what you want. (Everything will be pixel-perfect, too.)
For instance, if you want a Font Awesome icon that's not in the project, you can contract me out directly. You'll be able to do this for all of the paid icon sets: https://fonticons.com/sets/premium. Right now, we've got 2 gorgeous icon sets from Zach Roszczewski. You'll be able to contract him directly to get any icons his set might not have yet. (If you all don't know Zach, he's a particularly phenomenal icon designer. He did the Airbnb icons that are so gorgeous. The dude's seriously talented.)
Fiverr was overly saturated with more established artists. It was nearly impossible to find my listing organically.
Freelancer was filled with low quality / undervalued offers and spammy users. You can spend a lot of time fulfilling an offer just to wait weeks to find out if your work is selected amongst the other submissions.
The services seem good if you need some cheap work done, but felt pretty bleak from an artist's perspective.
