Probably not common in the US, but you can always try to negotiate with your existing employer. Networking should help here too.

If you live in a place with low cost of living, freelancing must be the most straightforward way to land a part-time job eventually. Some (obviously small) companies just don't have enough work (and money) to hire full time. Some of them go to Upwork or Toptal. I'd like to know what are other ways for them to find talent (except job postings of course).