Pool stores sell Liquid Chlorine, usually in 5 gallon drums called "carboys". I prefer the square carboys to the round ones. They don't roll when you lay them on their side.

There's usually a $6 or $7 deposit as they are reusable. I've been changing over my 3 for several years now. Look for the basic LC price to be between $16 and $20 (good bargain -- pretty expensive). You'll need a spigot to dispense the stuff easily--they are $4-$5 but last for years.

If you have the option, find the place that sells the most of this stuff. That means they turn it over quickly and it doesn't have time to degrade. The place I get it from goes through several pallets a week, and even though they say it's 12.5% (just double ultra bleach) I find it tests out at 14% usually. (To test, add 10ml of LC to 10 liters of water--that's 5 soda-bottles--in a five gallon bucket and test the water like you would your pool. The resultant FC amount is equal to the concentration level).