Hi Mike. I was going to say the same as VE3EAR, find a local motor rewinding shop theywill have high voltage varnish but this usually requires baking overnight, they may also have cambric tape, I was
Mike - 3M / Scotch makes a woven fiberglass-cloth tape (#27 ?) that Scott and I have used on previous output transformers...it's not cheap but is perfect for this purpose. Steve / VE7SL Further to my