Because the software is abandonware, you cannot buy it new. However, for legal license transfer:
A QuickBooks 2003 key code consists of 16 characters, divided into four groups of four characters each, in the format: XXXX-XXXX-XXXX-XXXX. quickbooks 2003 key code
If you have the original disc, look for a brightly colored sticker on the back of the white or yellow sleeve. This is the most common place for the 15-digit license number and 6-digit product number. The "F2" Trick: Where to Buy a Legitimate QuickBooks 2003 License