i recently made some changes in my breaker panel at home. It has a challenger breaker panel using c style breakers. The challenger product line was purchased by cutler hammer (eaton).
My research showed that the cutler hammer br series breakers are direct, approved replacements for the c series. There is a document on the eaton web site that validates the ul listing and the use in challenger and other panels.
Both lowes and home depot carry the br series. The breakers even have both br and c series part numbers printed on them, i.e. Br2100/c2100.