Do you mean by online or locally?
I wanted to buy from Greg Watson's website but after shipping/handling, it cost too much to ship to Canada. So I did some research and found local stores that sold dry ferts.
For locally, google "hydroponics" and your city name to find stores that retail and specialize in hydroponics. You'll find that there's a lot of equipment and other stuff that are perfect for planted tanks. For dry ferts, it might be a little harder. When I went to one in my area, I asked them for KH2PO4, KNO3, trace elements, and the person looked like she didn't know what I was talking about, probably cause not a lot of people go in looking for that kind of stuff. But after some thinking and some consultation with other workers, she found the chemicals + lots of other ones that might be useful in stock somewhere in the back.