Thread sizes below 1/4 confusing

try Restoration Specialty Supply.
Look through their pdf catalog and if you can't find what you need, call, send an e-mail or both.

There are several differences between "Metric" and "American Standard."
I use quotes because these names are generally what people use, but if you're in the industry, then you'll be even more specific.
One difference is thread form. This is the shape of the thread in cross section, and the hieght relative to the diameter, etc.
Another is the dimensions - obviously.
And maybe almost as obvious is the designation method. Standard used nominal diameter and threads per inch. Metric uses diameter and pitch.

You asked about under 1/4" designations.
IIRC correctly these originated with gage sizes. There's a formula you can use to figure out the diameter, if you remember it.
diameter = .0600 + .0130 x size

Try it out.
Size 0 is what diameter? 0.060"
How bout a 10-32?
.060 + 10 x 0.013 =
.060 + .130 =
0.190"

For 0 and 00, use -1 and -2.