The aptly named State of New York Stock Transfer Stamps has high-resolution images of over 300 varieties of these stamps. Content is nicely indexed, with examples of key identification features, and usage examples.
Rob Henak’s State Drug Tax Stamps site is a gateway to controlled substances tax stamps from the lawyer who invalidated Wisconsin’s controlled substances tax!
Eric Jackson is the world’s leading philatelic firm dealing in United States and Canada revenue stamps and related literature.
The Catalogue of the Crawford Library of Philatelic Literature at the British Library contains indexes to, and searchable documents digitized from the major philatelic library built up by the 26th Earl of Crawford. This library was willed to the British nation by Crawford upon his death in 1913. Some documents refer to U.S. state revenue stamps.