That's interesting, I dont recall seeing any tanks that were less than 10g being sold alone... It may be because tanks of that size are marketed towards younger children/inexperienced people that may not know about the nuances of equipment and lighting and just want something all-in-one.
Found this on Amazon:
My Petco and petsmart have 2.5 gallon all the way to to 75g available as bare tanks... Maybe a hood on some, you might have to ask for them. They're usually in a different aisle then where the tank stands are