http://www.historicaerials.com/
Howdy! I don't think John/Lowjiber saw your question, so I'll try to help out
I browse this site often to do map overlay ( free) You just have to subscribe if you want to buy any imagery/maps. Did you try it out? I love this site, I could compare maps and topos forever!
www.historicmapworks.com is great for older county maps but unfortunately my area doesn't have overlays (yet) they tell me to keep checking back and I have requested. But I joined and bought an 1877 map ($9.99) of my area and use it to make notes/highlight areas I want to research and hunt.
... and finally I have my own REAL treasure map!