Starboard is also know by its generic name - high density polyethylene (HDPE)
its the same stuff they use to make cutting boards for commercial kitchens.
I am not sure where you can get it in sheet form - you might be able to do a special order from Canal Plastics, but its probably going to work out a lot cheaper if you get it from a catering supplier on the Bowery. You might need to get two boards, you will have to see how big they come, but two boards makes it easier to install than one big one.
I would recommend sealing it to the bottom of the tank with silicon. that way you dont have any worries with detris getting trapped under it and pushing your nitrates up.
As far as needing sand to create a place for bacteria to colonise, that's what live rock is for. In my opinion, I found having a bare bottom with starboard made it much easier to keep my tank clean which resulted in much happier livestock, and the white of the bottom help reflect light back up into the tank, protected the bottom and allowed a substrate to be colonised by zoo's and other softies that would not attach so readily to glass.