I got mine through mail order in the US. (
www.petsolutions.com) I also ordered most of my dry goods through them when first setting up my tank just over 6 months ago.
They had a deal on the Seachem products at the time, and I got two small containers of Reef Advantage Calcium and one of Reef Builder. I still have one full Advantage, and half a container of the others. I use them in my dozing once every week two.