On the internet! :joke:
Or here.
http://www.thatpetplace.com/Products/KW/W52AX/PgNo/1/Class/Pond+Planting+Soil/Shopay00.aspx
Or in many garden shops. I find that most of them sell it in one variety or another.
In my area there is are a couple of chains that sell it either under Schultz or Turface.
I suggest you look locally for the stuff as it will cost you a small fortune in shipping to order on-line.
BTW, I have a 55 with Flourite and a 55 with Schultz and both tanks are thriving. The Schultz is a bit lighter and a little more difficult to keep plants down in when new, but as it loads up with mulm and bacteria it's weight increases and it's great stuff.
I wouldn't use anything else in a large tank.
Len