Ten bucks for them isn't too bad, with shipping included, if they are NOS and not just used up, tired old ones (or UOS, Used Old Stuff)
It may be tricky finding a truly matched pair, and they can have a fairly wide range of actual (measured) operating characteristics, but I've never had a problem with that, they seem to work well, anyway.
Just hunt around on e-bay, and if there are and hamfest / ham radio flea markets or antique radio swapmeets in your area, scope them out, sometimes these tubes can be gotten for a buck or two - other than the Darling circuit, they have never had much widespread audio acceptance or usage, as they were primarily designed as RF oscillator and Low Powered (5W or less) Class C RF amplification.
/ed B