Th closest to an online catalog are some dealer websites because they sell using Scott numbers. The two that I know about are Brookman's and Mystic. Brookman's price lists are extracts from their printed catalog so it is organized like a catalog. They show most collectible stamp include minor varieties. The Brookman printed book may be more practical than Scott.
http://www.brookmanstamps.com/pricelists.htmlExample screen shot of a page.

Mystic Stamp also list most major issues at
https://www.mysticstamp.com/