I bought a few electron microscope apertures of various sizes from Earl Johnson on the old f/295 forum. Since that forum closed I've lost track of Earl. But as pinholes the EM apertures are superb, being laser drilled in very thin copper sheet. I've built three pinhole cameras using them, a 2x3, a 5x7 and a 5x12 with curved film plane. Each pinhole was $1 each posted. The only drawback with them is that they're only 3mm diameter which makes them very fiddly to mount, but not impossible. I believe James Guerin of Reality So Subtle sells these same pinholes mounted in larger discs. It's worth a look at his website:
https://realitysosubtle.fr/
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