I have found it on Amazon, target, Walmart, (grocery stores and online) some flavors may be harder to find than others but I can usually find something I like in stock locally. One bottle is about 5.99, but a little goes a long way.
I also use a zero water filter. The slightest chemical taste really makes it hard for me to drink water, so having crisp clean water and then adding a little flavor sometimes has made it much easier to get 64 Oz a day in.