Usually, they find you ;-) I’m only slightly kidding here.
This is a very crowded market and if you can avoid working with them it’s probably best to do so (for pretty much the same reasons as is avoiding working with Upwork).
There are a few really good ones but most of them just try to sell developer hours as a commodity.
Where you can find the good ones depends a lot on where you’re located. There are freelancer forums (on LinkedIn, for example) where recruiters usually advertise new projects.