If you're looking for inexpensive replacements, try 2 liter soda bottles. They're tougher than
milk jugs, and I'm pretty sure the plastic won't degrade as fast.

A slightly more expensive alternative would be these.
I bought six of these for water during the hurricanes this year. Solid construction, stackable, integrated
spigot, each holds 7 gallons. Nice. $12.50 each, but no worries about rotting out from under you.

(Don't know if you get a break on the shipping, but REI gets them from the manufacturer in boxes of
six. I know, because REI just stuck a shipping label on the factory box when I ordered my six.)