A lot of people say that the rock will absorb phosphates/metals, etc. if you use tap, and then release it later. I have no idea if that's really true. We used tap for the first few months of our tank, and haven't had huge algae issues (other than bubble algae).
Sorry, I have no idea on the eBay sellers. I think a local club/forum would be a good bet. We had our LFS get us a couple of boxes in with their weekly order, so we got it at about $3 a pound (uncured). The boxes of Fiji are usually 50lbs each (gross weight), so about 40lbs of actual rock. You'll usually get better deals with whole boxes.