The Perfin Society aims to record all perfins on GB stamps, their period of use and identity of user. Many perfin users were also commercial overprint users. Indeed, a number of companies using overprints in the Victorian period switched to perfins, presumably as this was more cost effective, although many companies and organisations used both methods concurrently. Members of the Perfin Society receive a bulletin every two months and are able to bid in Society auctions which quite often also contain commercial overprint material.
A Catalogue of Foreign Bill Stamp Overprints and Handstamps by Jeff Turnbull offers a detailed study of these overprints with plenty of stamp images.
A Catalogue of Perfin Contract Note Stamps by Jeff Turnbull also contains a section on overprinted Contract Note stamps.
The Revenue Society promotes the research of revenue stamps from around the world. The Society holds regular auctions, occasionally containing commercial overprint material.
Underprints offers a study of underprints and protective overprints on GB stamps. A number of companies engaged in underprinting went on to use commercial overprints.
GB Overprints Society studies overprints on GB stamps. In the main, these are GB stamps overprinted for use in other countries.
IB Red Guy is a revenue stamp dealer who has a gallery of commercially overprinted stamps on his website, as well as images of many other revenue stamps from around the world.
Precancels of Great Britain offers a detailed study of GB precancels as well as telegraph stamps from around the world.
The Cinderella Stamp Club is interested in revenues, fiscals, local stamps, railway stamps, telegraph stamps, charity seals, labels of various kind, and other items which are regarded as the cinderellas of philately.
Commercial Overprints of Australia by Dave Elsmore offers a comprehensive study of perfins and overprints on Australian Revenue stamps.