I want to find freelancers for a project, but I don't know how to find them.  There are plenty of articles about how to become one, but I can't find any on how to find some.

You can peruse the offerings on GameDev's Contractors page: https://www.gamedev.net/contractors/

There's also a listing on Gamasutra: https://gamasutra.com/contractors/contractor_display.php

And you can scroll through the local companies/contractors listed at https://gamedevmap.com/

And you can use Wikipedia's https://en.wikipedia.org/wiki/List_of_video_game_developers and https://en.wikipedia.org/wiki/List_of_indie_game_developers

I'm going to sticky this to the top of the forum.

Upwork is another option. 

https://www.reddit.com/r/gameDevClassifieds/wiki/index

https://polycount.com/categories/freelance-job-postings

The best way to find a developer is probably to look through linkedin profiles or toptal. However, I'd also recommend you consider such an option as a dedicated team. Such a team of developers can help you complete both short-term and sustainable long-term projects. Moreover, their greatest benefit is that engineers are already certified and examined by the development company, so you don't need to worry about their expertise level. 

Finding them is easy, but choosing the right one is hard. I can only suggest to spend enough time and don't pick the first one who replies to your offer. Make sure that your needs are defined in detail.

This is a great summary of tips (not only for artists):
https://meltdowninteractive.com/2019/01/27/tips-for-hiring-and-working-with-artists-in-game-development/

You will find plenty of experienced freelancers on Upwork and Fiver with reasonable rates.