It seems like I once saw the best range map at a Jeff Biggs website, but I no longer find it.
Here are my two current suggestions, both from a Google search:
http://www.ratsnakefoundation.org/index.php/guides/locality-guides/texas-ratsnakes
http://www.google.com/imgres?sa=X&biw=1024&bih=673&tbm=isch&tbnid=Fx2CsjMkeWje4M:&imgrefurl=http://www.duvalschools.org/sandalwood/herpetology/Species%20Accounts/Snakes/texas%20rat%20snake.html&docid=k-1P4YIppMj77M&imgurl=http://www.duvalschools.org/sandalwood/herpetology/Species%2520Accounts/Snakes/texas-rat-map-large.gif&w=577&h=792&ei=nME4UvTMOoeTiALQqIHIAw&zoom=1&ved=1t:3588,r:53,s:0,i:248&iact=rc&page=4&tbnh=194&tbnw=142&start=46&ndsp=18&tx=79&ty=96
Ameron
Portland/Vancouver
1.0 Elaphe schrencki (Anerythristic)
1.0 Pantherophis guttatus (Miami phase)

