Stamp #088 is obtained by beating Travis in a single lap race around Cloud Hill. There are arrows and barriers around the racetrack to make sure you don't take a wrong turn. You can, however, fall off the Rainbow Bridges so be careful of that. Travis is a fast car so Jet Turbine are recommended for obtaining this stamp. You can race Travis again but there is no reward for beating him twice or more.
When the race starts, turn around and go to the end of the longest rainbow. Push Travis off and you'll win - whether he already "has" or not.