When Bottlehead revised the SEX 2 to the SEX 3, they bought up all the 6FJ7s they could find. Even now, 6 years later, vendors I'm aware of still have no stock of 6FJ7s. Even ebay comes up short: a few singles, but no pairs.
Presumably, there are lots more 6FJ7s out there, but sellers probably don't realize there's a market.
Given that, $13 for a tested NOS 6FJ7 might be about as well as you can do.