Not that good a thing to be doing as each such project potentially balloons into drama, dealing with each new client and their often confused or inexperienced expectations will be its own hassle, etc. Working full time or having a few long term clients is a lot better, as is getting leads through people you know if you have some special expertise in something.
I've heard people suggest looking for bug bounties. I haven't tried that myself so I'm just relaying the suggestion.
Front-end web stuff is its own circle of hell but back-end is just like any other programming, so you could look for that.
I mentioned the "freelancer" HN thread a while back, but it's mostly people looking for work: https://news.ycombinator.com/item?id=44434575