There are a few different lengths, but most are 3/16" diameter. I have looked all over, and it is difficult to get long, 3/16" in black AND stainless (like what you would need for riveting a chinpad.
You are likely to find black, but they are probably going to be black oxided steel. Please let me know if you do end up finding black, stainless 3/16" long rivets somewhere.