Links to websites which may be of interest to commercial overprint collectors :
The Perfin Society : www.angelfire.com/pr/perfinsoc/
GB Overprints Society : www.gbos.org.uk
The Revenue Society : www.revenuesociety.org.uk
Precancels of Great Britain : www.gb-precancels.org
Protective Overprints and Underprints : www.underprints.com
www.ibredguy.co.uk – a revenue stamp dealer who has a gallery of commercial overprints on his site.
No links to any online stamp dealers as I have yet to find one who sells commercial overprints. However, Perfin Society auctions feature overprint material quite regularly. Otherwise the best sources on the web for overprints are www.ebay.co.uk and www.delcampe.net