That's right - LW aren't linking to 4.18.8 on the site any more.
I'm not a Mac user, but AFAIK you should be able to use a 4.16 version with Tiger. Check out LOTR's advice in this thread:
Downgrading to LimeWire 4 from 5 & resuming incomplete downloads
There's a further link there to download a 4.16 version & info for installing an earlier LW
If you really want 4.18.8 rather than 4.16, send a PM to LOTR using this link:
http://www.gnutellaforums.com/privat...=newpm&u=26490
and see whether he can help you.