Metric threads

1/4" does not equal #6-32 they are completely different threads and sizes.
1/4-20 and 1/4-28 are possible # 6 is really small.
#6-32, #8-32, #10-24, #10-32 are all "inch" sizes plus some more

 

If there is an ace around you they usually have a pretty decent selection of nylon hardware. I think the big box stores have some as well.

if its listed with an M such as M6 or M8 its metric.
If its listed with a #sign or nothing its standard. #8, #10, or 8-32, 10-24 etc. I deal with taps and threads everyday and I can see where it could easily get confusing especially depending on how the catalogs list things.
Drill sizes are just as bad if not worse. Number drills, letter drills, fractional drills, metric drills....

 

Not that I have seen.