1 inch unthreaded EC90

[tommyw]

guys, this is really very helpful indeed. thanks, seriously.

 

[tonyf39]

You could look at 2nd hand but make sure you know beforehand what length steerer you need taking into account the stem clamp depth and any spacers to give you the height needed.
Certainly older Principia frames took a straight 1" fork (unlike the later ellipse variants) so that might be a source, I'd also check on the LFGSS website as there's huge amounts of stuff for sale there.