I ordered mine when I was using it a few months ago. I couldn't find it at any local stores. I think Lowes or HD have some stuff, but it is more of a polish than a Novus 1,2, or 3 like you will need if the scratches have any depth at all.
P.S. Use a power buffer - I tried doing it by hand for about 12 hours and was getting nowhere.
I skipped the sanding part. I couldn't bring myself to purposely rub sandpaper into my aquarium. I used #3 first until the scratches were very light, then switched to #2 until they disappeared. I used a buffing pad (several) the whole time.
Your sander should work fine. I used both a foam buffing pad and some terry-cloth type buffing pads. If you want to speed things up, you could try the sanding in an inconspicuous area (I might try it next time). I couldn't find buffing pads made to fit my size of sander, so I just got some that had a fuzzy exterior that would stick to the velcro of the sander. Its not a lot of fun, but the results are definitely worth it. Also, I can guarantee that you will be extra-careful about not scratching it in the future!