Anything that fits either a 2006-2008 or 2009-2015 15" MacBook Pro ought to work, as the dimensions are quite similar to those of the 15" PowerBook G4 in my experience (this is especially true of the 2006-2008 MBPs, as they used laregely the same case design as the last PowerBooks).
Cases/sleeves made for recent 15" PCs might be a bit harder, because they're mostly designed with a wider aspect ratio (16:9 vs. the PowerBook/MacBook's 16:10), so the fit might be a bit tight.
I hope this helps!
c