Lots of startups do this, because it's difficult to commit to hiring full time and they often have a burst of work needed early on.
A lot go through some marketplace like Upwork, but contractors usually get the worst deals there, because you're competing with a lot of others.
80% of my contracts came from two people. One was an early stage VC, the other ran an agency that did design but didn't do code. So instead of building relationships, just piggyback on those whose full time roles are doing it, but won't compete with you. Focus on being the best at your work and people should search for you.
