Purchasing tubesets from the amp manufacturer will generally mean overpaying, which can amount to a lot with amps having as many tubes as these.
The SED 6550C is VTL's stock output tube for these amps and a fine choice, though I've also liked the results from using the drop-in compatible Electro-Harmonix KT-88EH for a less-'tubey', cleaner sort of sound (I haven't yet tried SED's own well-regarded version of the KT-88, nor the JJ/Tesla some have spoken highly of). Based on past experience with other power tubes I'd avoid the cheaper Sovtek or Chinese versions, but haven't heard those 6550's/KT-88's specifically.
The 6350 drivers are normally available made only by US Sylvania and are not costly.
Those are the tubes you should replace at 3-5 year intervals due to wear (depending upon use). The input amplifier 12AT7's may last longer than the driver and power tubes, but here it can pay to experiment with premium NOS varieties, as none of the newly manufactured examples are as good as what used to be made back in the day, and this tube is very deterministic regarding the overall character of the amps' sound.
But I won't open that can of worms here (you should do a forum search on the subject), and if you just want an inexpensive 12AT7 to drop in and see if the performance of your existing input tubes can be improved for the time being, I would recommend the commonly available US JAN Philips/Sylvania/GE varieties or Yugoslovian EI's over the current Russian, Czech, or Chinese versions.
The best pricing I've found on the more costly and numerous power tubes is at Triode Electronics Online, while the EI input tubes are sold at TubeDepot.com. BTW, this is just my experience, but I haven't found it cost-effective or failure-preventative to pay a premium for new power tubes from a reseller charging extra to burn-in, pre-test, and match the power tubes beyond what the Russian tube factories already do as standard. (Close output tube matching is not needed with the VTL's individually-adjustable bias trimpots, though your input tubes should be bought as matched pairs to ensure an even gain balance between channels.)
Before installing your new tubes, remember to turn the bias pots on the output tubes all the way down first, then turn on the amps and let them warm for a few minutes, then gradually bring tubes up to spec on the meter as the they continue to warm, rebias them again after 10 more minutes, and then again 30 minutes later, plus check them again after 3 days and once more at 2 weeks, by which time they should have stabilized enough to only need a check every 2-3 months or so.
The SED 6550C is VTL's stock output tube for these amps and a fine choice, though I've also liked the results from using the drop-in compatible Electro-Harmonix KT-88EH for a less-'tubey', cleaner sort of sound (I haven't yet tried SED's own well-regarded version of the KT-88, nor the JJ/Tesla some have spoken highly of). Based on past experience with other power tubes I'd avoid the cheaper Sovtek or Chinese versions, but haven't heard those 6550's/KT-88's specifically.
The 6350 drivers are normally available made only by US Sylvania and are not costly.
Those are the tubes you should replace at 3-5 year intervals due to wear (depending upon use). The input amplifier 12AT7's may last longer than the driver and power tubes, but here it can pay to experiment with premium NOS varieties, as none of the newly manufactured examples are as good as what used to be made back in the day, and this tube is very deterministic regarding the overall character of the amps' sound.
But I won't open that can of worms here (you should do a forum search on the subject), and if you just want an inexpensive 12AT7 to drop in and see if the performance of your existing input tubes can be improved for the time being, I would recommend the commonly available US JAN Philips/Sylvania/GE varieties or Yugoslovian EI's over the current Russian, Czech, or Chinese versions.
The best pricing I've found on the more costly and numerous power tubes is at Triode Electronics Online, while the EI input tubes are sold at TubeDepot.com. BTW, this is just my experience, but I haven't found it cost-effective or failure-preventative to pay a premium for new power tubes from a reseller charging extra to burn-in, pre-test, and match the power tubes beyond what the Russian tube factories already do as standard. (Close output tube matching is not needed with the VTL's individually-adjustable bias trimpots, though your input tubes should be bought as matched pairs to ensure an even gain balance between channels.)
Before installing your new tubes, remember to turn the bias pots on the output tubes all the way down first, then turn on the amps and let them warm for a few minutes, then gradually bring tubes up to spec on the meter as the they continue to warm, rebias them again after 10 more minutes, and then again 30 minutes later, plus check them again after 3 days and once more at 2 weeks, by which time they should have stabilized enough to only need a check every 2-3 months or so.