If you have a swimming pool installation/refinishing company, or a concrete floor surfacing company local they can invariably get it. I bought my first couple bags from a place called 'Arizona Polymer Floors', and the remaining bags from 'Mason Mart' here in town. The stuff was originally designed to be mixed with concrete as a solid colorant (don't let that scare you...there's hundreds if not thousands of people using this stuff in fish tanks...it doesn't dissolve or anything of that nature...and is perfectly safe for fish).
The 3m website has a distributor locator that is pretty handy as well...and for 18$ a 50lb bag for an incredibly pure, heavy black substrate...it would probably be worth it to ship.
Cris
