I don't know why it's so hard to find the manga, even in the U.S. Just check out Kinokuniya's Bookweb site and it shows a bunch of them.
US West Coast:
http://bookweb.kinokuniya.co.jp/indexohb.cgi?AREA=02
US East Coast:
http://bookweb.kinokuniya.co.jp/indexohb.cgi?AREA=03
Granted the page is all in Japanese and you need to type the titles in Japanese (just copy and paste: ひぐらしのなく頃に ), but all three Higurashi mangas pops up with in stock availability at their respective stores.
Either that, or just visit the nearest Kinokuniya in your area:
Seattle store
525 South Weller St., Seattle, WA 98104
Phone: 206-587-2477 Fax: 206-587-0160
E-mail:
seattle@kinokuniya.com
Portland store
10500 SW Beaverton-Hillsdale Hwy., Beaverton, OR 97005
Phone: 503-641-6240 Fax: 503-643-1059
E-mail:
portland@kinokuniya.com
San Francisco store
1581 Webster St., San Francisco, CA 94115
Phone: 415-567-7625 Fax: 415-567-4109
E-mail:
sf@kinokuniya.com
San Jose store
675 Saratoga Ave., San Jose, CA 95129
Phone: 408-252-1300 Fax: 408-252-2687
E-mail:
san_jose@kinokuniya.com
Los Angeles store
123 Astronaut E. Onizuka St., Los Angeles, CA 90012
Phone: 213-687-4480 Fax:213-621-4456
E-mail:
los_angeles@kinokuniya.com
Costa Mesa store
3030 Harbor Blvd. #G-3, Costa Mesa, CA 92626
Phone: 714-662-2319 Fax:714-662-2474
E-mail:
costa_mesa@kinokuniya.com
New York store
10 West 49th St., New York, NY 10020
Phone: 212-765-7766 Fax: 212-541-9335
E-mail:
nyinfo@kinokuniya.com
Palisades Center store
3360 Palisades Center Dr. West Nyack, NY 10994
Phone: 845-353-6600 Fax: 845-353-0407
E-mail:
pcinfo@kinokuniya.com