How can I buy a new 8GB iPhone 3GS that still has iOS 4?
Firstly im not sure the phone needs to be jailbroken and if a contract is up the carrier is required to give instructions on how to unlock.

The packaging is not different.

Not all 3GS models are bundled with iOS5. I would look at some websites that do not sell a lot of these phones, ask shops how long they have had the particular model in stock, if the had them for over 2 months then you're in luck.

You could also look to buy a second hand model, which hasn't had an update.

Can I ask why you do not want iOS5?