You can get the later books from other outlets like Amazon. Scholastic is colorizing all nine of them and I think they're up to #5 now (they're releasing them every six months), though you could also buy the whole darn thing in its original black and white as well.
Comments
http://search.barnesandnoble.com/booksearch/isbnInquiry.asp?z=y&EAN=9781888963144&itm=1
NOTE: This is the original format. It is NOT IN COLOR
http://www.boneville.com/2007/05/07/bone-volume-6-old-mans-cave-ships-end-of-june/
That's the One Volume Edition.