I hear you can use 6N6P as replacement, and they are significantly cheaper!
Some claim they sound better too. (after some break-in or getting used to them, whichever you believe

)
They are a drop-in replacement in many designs, and if they are not suitable for a particular design they will not break anything, just sound bad.
I'd say they're worth a shot, and I'm considering buying a bunch before they shoot up in price as well.