Free: Contests & Raffles.
I couldn't stomach spending the money on some MRS bags, so I bought the cheap 2 gallon clear plastic collapsible bag with a red/white screw on nozzle. Works great and less than 10 bucks. It's lasted 3 seasons and counting.
Franzia boxed wine bladders. 5 liters for a couple ounces of weight. Folds up into 3inches by an inch and a half. Pretty durable. Using one without a leak for several years.
I reuse 2 liter soda bottles. They weigh next to nothing and don't cost anything.