A quick scan of eBay shows lotsa choices, both used, or new with
warranties. One of them is even starting at $1.00! The matching
sockets are also there as well, some for as little as $25.00. The
H3 version doesn't even need a socket -- it has flying leads.
Eimac 3CX10,000A7 Linear RF Power Amplifier Tube 8160 : 290073180762
NEW!!!! EIMAC/VARIAN 3CX10/000B7 AMPLIFIER TUBE : 160077347739
Eimac 3CX10000H3 3CX10,000H3 RF Vacuum TUBE : 140076841087
I've read that the tetrodes are a little less convenient, but still
usable by connecting the grids:
Used very good Eimac 4CX5000 Tested : 110082360349 (starting at $1.00)