I work at the Indypendent newspaper, and we want to run this map comparing free states and slave states from before the Civil War to the current division between red and blue states. We want to make sure the information is accurate before printing it in the paper!
Here's the link to the map.
The Library of Congress American Memory Project has a few maps you might find useful.
Bacon's steel plate map of America, political, historical & military(1863): "Federal free states" are outlined in red, "Federal slave states" are yellow, "Federal territories (free)" are orange, and "Confederate states (slave)" are green.)
Colton's map of the southern states … (1862): Map is colored to show "free, or non-slaveholding states" (pink), "border slave states" (yellow)…)
Slavery and Emancipation in the United States, 1777-1865 (Ancestry.com) National map of the United States showing the progress of slavery and the abolitionist cause, 1777-1865.
Slavery in the American South: 1790-1860 (interactive from University of Oregon)
Compromise of 1850: Status of Slavery (interactive)
Kansas-Nebraska Act: 1854 (interactive)