Hi Tokyofun,
We did a quick search and couldn't turn up any sites that explicitly accept non-Japanese people.
The usual escorts that we know aren't pregnant of course... but there a number of Japanese shops that have pregnant women or post-pregnancy women that are actively milking.
Here are some examples of those shops:
http://www.milkybaby.jp/ -- Offers both types.
http://www.bonyu-fun.com
If you can speak Japanese, it's worth seeking out shops that offer these services. If you want to do a Google search for these shops, then use the terms: 母乳風俗 (Bonyū fūzoku)
Good luck in finding what you are looking for!