You came to the right place!

Once you get into varieties, then you really have to go to rather specific specialist sites. So you'd have to tell us which countries/areas in particular. For example, classic US, then one good site is
www.1847usa.com. Unfortunately, Amos in all their manifest wisdom, had a lawyer issue a cease and desist order, so the site can no longer use Scott catalog numbers.
You can ask here, or you can post a request for stamp links for specific countries/times/areas and I'm sure members will be able to either answer or put up some useful links.
[EDIT: corrected typo in link (thanks, wt1!)]

k