I don't know much about lion board, but I did a quick googleing and theres few things I think I found out:
1. Lion Board is called Lion Board.
2. Tokyu Hands seems to be the prominent place that sells it.
3. Lion Board seems to be a specific board that Koyo Sangyou Corporation makes.
Lion Board's page:
So... it seems like it's only sold in Japan. Or atleast it seems as though it would be really hard to find it here in America. ^^;;
You seem to be very good with Japanese so you can probably read it, but it seems to be a "Plastic Foam Board." The tutorial site that I pasted below says that "It's easy to cut with scissors and razor, and it is also easy to glue together. Easy to make it round or sharp. Very light and also flexible."
Some Cosplayers site about different boards:
Lion Board Tutorial Site: http://www.umiu.com/cosplay/const/lionb.html
I probably wasn't much of a help. Good luck!
Oh, and if you can't find Lion Board, there was a site that seemed useful: http://amethyst-angel.com/armormaking_materials.html