It is highly recommended that you use a separate version of Re-Volt dedicated to the online races, as the custom content is often updated (and the content can differ from the original). If you don't have the game yet, you can find how to install it on this page. Afterwards, make sure to download the custom content listed below.
Alternatively, you can download pre-packaged versions of the game, which include everything that you need (last updated on 2 February 2017):
If you don't know which one to get, click on Win32. The download button is on top-right.
This pack includes various classic tracks, most of which fit in with the Re-Volt theme.
This currently unbalanced pack includes various cars that stay true to the Re-Volt theme.