Those fitting nuts are still readily available. If you can't find them at your local hardware or plumbing supply, McMaster Carr has them. https://www.mcmaster.com/#standard-single-sleeve-compression-tube-fittings/=1a9o7zr (link doesn't zero in on the item - 1/8" size is item # 50815K102)

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.