you'll probably be disappointed that they'll not deal with foreigners, however the good news is that there is a high chance some of these items will be available on Rakuten http://global.rakuten.com/en/such as this company http://item.rakuten.co.jp/yminfo/10000827/ they seem to sell very heavy rainsuits, but quite expensive at 12,000円 ($100 // £65)
This site sells very interesting rain suits in 7 different colors. These are based in Fukuoka in Southern Japan, so I'm guessing these are mostly for commercial fishing since that is a major port city. I can read some of the text, I have a kanji translator for the harder text. I highly doubt they sell these to foreigners though.
Nine-Coat Brand
45mm PVC rainsuits
http://www.kyusyuamagu.co.jp/index.html
This site sells plenty of amazing items, and like plenty of other Japanese sites - often uses sexy looking women to advertise their clothing lines.
They sell rainsuits, bibs, boots and some traditional food service kimono.
http://www.sinkou-marine.com/
