In my experience, we have hired remote coders exclusively through services like Upwork. And if there is anyone we really like to work with, we'll bring them in on a longer term contract based on their availability. But we have never, ever hired a part-time employee "from the street".

Part-time professional employees in the US are really disadvantaged these days because you still have to pay full benefits, so per-hour they are much more expensive than just hiring a full time employee and splitting their time between projects. Also, recruiting and interviewing is a huge time suck, especially just for someone who only does half time. So usually I have seen more contract-to-hire approaches.