Aaarghh....
Buying hobby branded sandpaper is like throwing money away.
Go to a hardware store or an auto parts store (eg. Super Cheap) and buy several grades of wet and dry paper. I use 400, 800, 1200 and 2000. These will cost you around $2.00(AUD) per sheet, each sheet being about 4x the size of the Tamiya sheets. You would be paying about $7.00(AUD) for a packet of 3 sheets of Tamiya sandpaper?
Wet and dry paper, as the name implies, can be used wet. When sanding plastic models or model parts, it should always be used wet. The water lubricates the paper and prevents overheating and melting when sanding, and prevents the dust from clogging the paper. Used wet, it will give you a smoother finish than when used dry.