the biggest my local fish store has for glass tanks in gallons is 300 gallon. The longest glass tank they have is a 265g, 7 feet long.
but, i think they have huge acrylic tanks made for people. They have a 1300 gallon saltwater display tank.
I saw a used 1800 gallon tank that had been pulled out of a failed restaurant listed on Craigslist last year. Did not have the money or the space, but it wasn't as crazy expensive as I thought. I am guessing one of the demo crew helped themselves to it before the building was torn down, and I figure I'll never see something like that again my lifetime. I am pretty sure the tank was glass if I am remembering correctly and I assuming it was customed made for the chain, but there were no details where it was from.