I see 3CX3000A7s around here go for much, much less than 8877s. Not sure why, maybe they were commonly used in broadcast applications by some local manufacturers who went away. But they sure are common at the local swap meets, often in sealed Eimac cartons. I wouldn't be searching "on line" for 813s and such; the bidding audience is too large, drives prices up to crazy. At the TRW Swap Meet it's common to find someone selling NOS 813s for $25-$30 each. I never bought any, but they're out there.