Congratulations seems a nice amp, a Triode/Pentode switch is great since Pentode are better sound than UL. Favorite shops are Upscale and VTS, Andy at VTS have large experience and answer customers by the phone.
You will find that the best ones have a price, KT88 GENALEX Made in England are famous great sound also KT88 Svetlana C-Logo, however beware of the fakes on these famous tubes and rebranding, buy on a reliable shop.
The original Genalex KT88 (England) are pretty much unavailable NOS now. If you could find a set - which is very unlikely - they would be astronomically expensive. What's more many say (including the well respected Jim McShane mentioned above) that the Genalex KT88 re-issue are their equal or superior. I've never heard the original but the re-issues are indeed very good. I did however prefer the Shuguang KT88 SC (previously sold as the Penta KT88 SC and now sold by TheTubeStore under their Preferred Label). The latter is a bit more refined than the Genalex. However I'd like to re-try the Genalex, cryo'd this time from Upscale Audio. That might close the gap between the two brands and Upscale has a stricter selection process than the other company.
The Svetlana "Winged C" line hasn't been made in several years (a shame as they - particularly the 6550 type - were superb) and the quality stocks of those tubes are long gone. The factory released some lower quality tubes at the end of their run and those are the only ones still floating around. Not recommended.
I haven't cared for the Tung-Sol re-issue power tubes. I've tried their 6L6GC and 6550 (tubes I normally love), and KT120. The first and third variety in a Rogue Cronus Magnum and the second in a pair of Quicksilver Mono 100s. All of these tubes had an overemphasized, even thick, bass. In fairness I don't recall hearing this in a pair of Rogue Apollo mono-blocks running KT120s I listened to one afternoon.
Aside from the above, it might be worthwhile trying the Sovtek 6550s from Upscale. They have been intriguing me for a while now as I always preferred the 6550 to the KT88. I'll buy them cryo'd if I do order them.
If your amp can take them and you don't need the extra power, the Genalex KT66 re-issue is excellent. Even better is the Reflector 6P3SE (6L6GC equivalent) which is getting a little hard to find now.