Where to find a 34mm internal headset cup?

28mm even

Not sure on what measurement you need and if it's internwl or external but this any good? Dimensions on the listing but they are decent quality at least..

https://www.ebay.co.uk/itm/154969095254?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=hq0hvvzgrx6&sssrc=2349624&ssuid=9RXnHnTiQQa&var=&widget_ver=artemis&media=COPY

remeasured and stopped being tired and mongy!

34mm top cup (got that)
44mm internal bottom cup for a straight 1 1/8" steerer

Is this any good?

https://www.merlincycles.com/fsa-no1n-integral-1-1-8-headset-58439.html

That's an EC34/28.6 | ZS44/30 headset. You sound like you need to get your [url= https://www.parktool.com/en-int/blog/repair-help/standardized-headset-identification-system ] SHIS[/url] together 😉

It's a minefield until you actually see it explained and it all makes sense. I think Hope and Cane Creek have a decent one page chart but can't find either right now.