I made my own stirrup leathers, so I have an idea of what you are looking for. Tandy Leather Factory - Waxed Linen Thread 25 yds. Tandy Leather Factory - Stitching Needles
As you will probably be reusing your old holes, I do not recommend a sharp needle, because you will very easily pierce through the leather and create a new hole where you don't want it. Make sure to pull your stitches tight too.
You will probably want to remove all of your old threads and restitch the whole buckle. If you want to go all out, I recommend a book called The Art of Hand Sewing Leather, but it is not necessary. You can find an instructional video on youtube on basic stitching pretty easy.
If you have any questions, try the Leatherworkers forum @ http://leatherworker.net/forum/
. There are saddle and leather tack makers on the website and they are pretty willing to answer questions.