I recently came across this website depicting the various stamps and souvenir sheets issued for The Salvation Army:
http://www.briefmarken.aaj.de/Although this website is based in Germany, it provides a detailed description of most every Salvation Army stamp issued, including catalog numbers for Michel, SG, Scott, Yvert, etc.
In fact, if you advance down to the individual country listings there is a link to "philatelic material" with images of most of the first day covers and related philatelic products that were offered at the time each stamp was issued.
This is one of the few topical resources I have come across that offers all of this information in one place.