Hydroponics and pretty much any good landscape supply company. Only problem is most places sell them in a 50 lb bag, or 25 lb at the very least. For most people, 1 lb is enough for an entire year. It's cheaper to buy them bulk/locally, but then you're stuck with a huge bag.
If you can organize a local club group buy and get people to buy like 2 lbs each, then locally is a great way to go.
Otherwise, you won't be able to beat Green Leaf or Aquarium Fertilizer.com