You can feed ordinary nori, just avoid the toasted and seasoned stuff - look in the sushi supplies area. Why no toasted? Beats me, that's just what I've read and I guess it stands to reason.
As for phosphates - I did read a thread on RC a few months ago where a guy actually did some experimentation with his tests and found a significant rise in PO4 when he put the stuff in his tank. I guess the obvious workaround would be to soak the nori in some water for a few minutes before putting it in the tank, AND PULL IT OUT AFTER A FEW HOURS. The stuff will start to fall apart, especially under current or if fish are being a little rough with the whole thing (as opposed to ripping bits off).
The stuff you are seeing is the same as the stuff in your LFS - it's just sheets of dried nori. Only thing I found was that at the end of the day, if you calculate the cost of the supermarket stuff per gram and do the same with the LFS packages, the savings aren't all that substantial. I thought that 12.99 was an idiotic price to pay for the stuff, but then realized that it was a 30 gram container, whereas the supermarket stuff was something like 4 bucks for a 12 gram bag.