I buy it at the duty free stores on the US/Canadian border. The Rainbow Bridge Duty Free store at Niagara sells it in the 1 liter bottle for $27.99 or two for $54 or three for $79. For the prices onliners want I would suggest a cheap flight to Buffalo, visit a casino, see the falls and stock up. One liter is always duty free but I understand they can charge a couple bucks for more. I got 5 liters on my last trip, declared them at the border and the agent didn't charge me a thing. I only got 5 because Southwest (and I think the other US airlines) only allows 5 liters in checked bags.
E-Bay has it for sale as a collectible bottle since it is not available for sale in the USA... Tipsy Gifts does have it but I don't know if you want to buy so much... (6-500ml for almost $300)... which is still like spending $100 for a liter. OMG it is good stuff.... why don't they sell it in the USA??