Konjac Shiratki

If anyone stocks them, it will be the supermarket on the corner of Rue de la Vierge Noire and Rue Sainte Catherine.
Which answers the second question as well.

Order from Amazon.co.uk and get delivered to your door, whatever commune you are in, for free if you pay over 25 euro

http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-ke...

Given it's Japanese, why don't you also look at the Japanese shops?
Tagawa in Ixelles
Haru-Chan in Watermael-Boitsfort
Ken-Chan in WSP
http://bxlblog.wordpress.com/tag/ken-chan/