There are (so far, it's still goin') 30 volumes out (Started in Japan in 1989-still being serialized in Young Animal Magazine (Hakusensha), and was licensed in America in 2003, by Dark Horse).
16 just came out like, two weeks ago in America. They come out about every two months (17 in May)
Okie Dokies, so; I's available in Borders (they won't have all of it, just like, 1,2,4, and 13. Yeah. Lovely) and most privately owned comic shops. It's really hit or miss for Barnes and Noble.
BUT! You can get all of them, all the time from Amazon.com, and they ship internationally.