A few weeks?
You still have lots of time! I did my sgp last minute (as in the weekend before it was due) and got a good mark
Don't buy one, the sgp teaches you a solid foundation on how to conduct your research (both primary and secondary). Some things may also be assessable in your exam (such as techniques, definitions, etc) and without doing it, you will have limited knowledge on these things.
I suggest you do your own sgp, however if you're stuck on what to do, you can look at some of the sgp bosers have posted on this site and follow a similar structure (with your own question and research)
http://www.boredofstudies.org/view.php?course=22 <-- geography resources page