I have the following document:
abc ccc ffffd zz
What you need is following-sibling XPath axis
following-sibling
//a/bb[text()="zz"]/following-sibling::cc[1]/text()
Test the Xpath here: http://www.xpathtester.com/obj/b55ec3ac-dfa4-4f44-81e8-f963ea4a0625