There's no official jar as there's no official release. The only "official" way of getting the SPGL is via CVS, for which you'll need a CVS client.
However, someone here might have an "unofficial" pre-packaged jar for you?
Otherwise, if you install Alien Flux via WebStart
you'll likely find a SPGL jar in somewhere like "C:\WINDOWS\Application Data\Sun\Java\Deployment\javaws\cache\http\Dwww.puppygames.net\P80\DMdownloads", but this is only the class files, and I've no idea how old it is. This should definitely be your last resort.