paint stores have 3.5 gallon buckets that you can fill to 3 gallon safely. They also have a scale on the side so you know exactly how much water you're making. Try a local Janovic, Ace Hardware or any hardware that sells benjamin moore paints.
Container store has some nice jugs.
They have the 1.5 gallon and a 3 gallon that work great for nano tanks.
Also have some nice flat sided ones.
I use the flat ones for my office tank.