Bana 20a, I've had great luck in going to antique stores for old maps. I've found 1920s maps of my state for, at the most for $8. Then there is ebay.....a 1912 Montana map ($7....I live in Minn, but visit Montana regularly), Minn maps from 1919 to 1960, and many other maps. DON'T buy maps that were ripped out of an atlas....they're next to useless.
Another source is your local museum or historical society. They have REAL maps of local towns and for around $1 they will give you a copy. I wish you luck.