During a regular check on the PassionForPuzzles web site I found out that the Puzzle Japan website does not longer exist (broken link).
You are kindly requested to visit the Nikoli website. I think this is strange because last month they gave free access to all their puzzles (like Sudoku, Kakuro and nurikabe) as posted by Puzzlinks and Zotmeister. So it is time to update your bookmarks. Happy Puzzling!
You can also buy other Nikoli Slitherlink puzzle books from Amazon.com.