Biker can be found on VILLAGE - RACE 3.
You can get this naturally by hitting or attacking him off his bike 5 times. This may take quite some time to do without a boost if you didnt know to hit him. Listen for laughter and you will know you are close to him.
To boost:
MODE: TIME TRIAL
TRACK: THE VILLAGE - RACE 3
TARGET: BIKER. Easy to find since he spawns right in front of you at the start of the trial
GOAL: HIT 5 TIMES OFF HIS BIKE. Hit him off his bike and wait a few seconds for him to respawn. Alternatively, do another lap to kill time and nail him again as you come around again