Hi,
a few ideas that comes to mind:
search for challenges or small project ideas online, solve them and add it to your GitHub portfolio somewhere. Example: https://github.com/florinpop17/app-ideas/blob/master/README.md
some websites offer you to solve challenges and get hired based on the solution you wrote, such as https://geektastic.com/, unfortunately I'm not familiar with any that offer React challenges in specific.
Find an open source project that is being written using React on GitHub.