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.)
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.)