In my own app, I'm using Semantic-UI as the styling baseline, especially for inputs. There's a number of React wrappers around Semantic-UI, such as
https://github.com/hallister/semantic-react and
https://github.com/TechnologyAdvice/stardust, but most of them create React wrappers around Semantic's jQuery-based widgets. I'm trying to avoid use of jQuery entirely, so I opted for
https://github.com/shinzui/react-semantic-ui-kit instead. Not as complete as the other libs, but a great focus on just wrapping the CSS/className aspects of Semantic-UI.
Beyond that, https://github.com/jquense/react-widgets is a great set of more specialized input widgets (dropdowns, datetime picker, numeric spinner, etc), and https://github.com/rofrischmann/react-layout-components is a good React wrapper around Flexbox usage.