The Sunn 40 watt (2xEL34) and 60 watt (2x6550 or 2xKT88) amps were based on Dynaco amp kits. The transformers in the early models were the exact same ones as what were used in Dynaco hifi's. Later, Sunn started buying transformers from Schumacher. The Dynaco transformer Sunns have lower B+ voltages than the later Schumacher ones. They both sound a little different as a result. I happen to like the earlier Dynaco transformer amps better, but since everyone hears differently you may like the later ones better. The Dynaco transformers have been available for a Long time from Triode Electronics....at least 15 years anyway, and they are exact clones, with the exception of the power transformer current rating being bumped up a bit since stock ones tend to fail over time. You can find them here.
Triode USA Tube Amp Kits Transformers Tubes Dynaco Upgrades and Parts
Mercury Magnetics makes good transformers at a premium price. If you buy enough of them then eventually your price goes down. The Triode Electronics transformers do the job just as well as what Mercury Magnetics sells, but they are a lot cheaper.