For this kind of specific dimensions, I would go the DIY route... Such big printers does exist, but are expensive.

You can go for a modified solidoodle with extended Y carriage, and find a bed that will be big enough. You could make an aluminium surfaced plate, and stick 4 bed heaters underneeth it, controled by 4 SSR on the same card output. But it will be a huge work. You'll have to go for a bigger rods dimension, 12 mm or 16 mm to carry that weight, and your machine will be slow.

Now with all that, you'll have to design and make work a new Z axis system, for that monster plate, not talking about the numerous powersupply you'll need to heat 800 mm of bed...

All in all, it might be worse considering slicing your prints into small ones that you can glue together...

+1
http://store.makerbot.com/replicator-z18 prints 18 inches, but costs $6,500. Pretty swanky though!
It sounds like you could do your project in 2 sections and just run a threaded rod through the stock and bond the two peices with some acetone if using ABS.

But if you wanted it printed all in one piece, then yes. It would be far cheaper to build your own printer.

You can go for a modified solidoodle with extended Y carriage, and find a bed that will be big enough. You could make an aluminium surfaced plate, and stick 4 bed heaters underneeth it, controled by 4 SSR on the same card output. But it will be a huge work. You'll have to go for a bigger rods dimension, 12 mm or 16 mm to carry that weight, and your machine will be slow.

Now with all that, you'll have to design and make work a new Z axis system, for that monster plate, not talking about the numerous powersupply you'll need to heat 800 mm of bed...

All in all, it might be worse considering slicing your prints into small ones that you can glue together...