You have a couple choices. You could find the Sid version here,
download it, and install it with dpkg -i.
A better solution which does involve adding Sid to your sources.list, is to correctly set up a mixed Lenny/Sid system
with the preferred release = Lenny. That way, you can get the occasional package from Sid, but you'll have to ask for it specifically. Normally any package you install or upgrade will come from Lenny.