If your looking for a quick printed invoice slip, why not head to your local stationery store, staples etc, they usually have pre printed invoice books with carbon copys etc that you can use
lol oh man, ok well if your stuck at home and dont fancy a drive/walk/or hobble to your local store, try a company called Viking direct, they're pretty good and will deliver to your home/work address.
Feel free to take the free trial of KashFlow (link in my signature, below) just to generate your invoice. If you want me to I'll then delete your account once you've got what you want.