Maps showing slave distsribution from Library of Congress and University of Oregon.

The Library of Congress American Memory Project has a few maps you might find useful. http://memory.loc.gov/ammem/index.html

Map showing the distribution of the slave population of the southern states of the United States. (1861) http://hdl.loc.gov/loc.gmd/g3861e.cw0013200

Bacon's steel plate map of America, political, historical & military. (1863)
http://hdl.loc.gov/loc.gmd/g3701s.cw0036800
("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)
http://hdl.loc.gov/loc.gmd/g3860.cw0025600
(Map is colored to show "free, or non-slaveholding states" (pink), "border slave states" (yellow)…)

source: Ancestry.com
Slavery and Emancipation in the United States, 1777-1865
http://www.ancestry.com/search/rectype/reference/maps/freeimages.asp?ImageID=427
National map of the United States showing the progress of slavery and the abolitionist cause, 1777-1865.

Source: University of Oregon
Slavery in the American South: 1790-1860 (interactive)
http://darkwing.uoregon.edu/~atlas/america/interactive/map17.html

Compromise of 1850: Status of Slavery (interactive)
http://darkwing.uoregon.edu/~atlas/america/interactive/map20.html

Kansas-Nebraska Act: 1854 (interactive)
http://darkwing.uoregon.edu/~atlas/america/interactive/map21.html