I have a html snippet that looks like this (of course surrounded by other html):
Header: Tex
Header:
Tex
This is untested, but you might be looking for simple_html_doms next_sibling() method.
next_sibling()
$html->find('p[class=finfot3]')->next_sibling()->innertext() should return the contents of the second element.
$html->find('p[class=finfot3]')->next_sibling()->innertext()