I often buy "common" notes, the best place is at a coin show. In fact, I have bought some of them from 2 of the regular contributors to this site.
I don't want the rare notes, except to auction them.
The common notes, like 1937 $1, $2, $5, $10, and 1954 issue same denominations, and 1979 $5, and 1971 $10, I buy them all the time, in large quantities. I often run out of 1989 $10's as well.
So, collectors, bring your odd notes to a coin show. There are dealers who will buy them.
Rick