Not strictly slalom, but every second Monday of the month there is a longboarding get-together in Yoyogi koen at 8 pm. The next one is tomorrow, December 13th.
More details here:
<a href="http://www.outdoorjapan.com/cgi-local/u ... =000870</a>
Longboarding in Yoyogi koen, Tokyo
Moderator: Tatsuya Takahashi