Hey guys, sorry for so simple a question.

I have a 27.2 mm seat post with a QR clamp & want to get a bolt style clamp. I cannot find a 27.2 clamp anywhere. Am I looking for the wrong size clamp, are clamps supposed to be sized larger than the seat post? Should I just be looking for a 28.6 mm clamp which seems to be everywhere? Again, sorry for the simple question but I'm lost on this one.
Thanks all.