3701 W. 12th Street
Topeka, KS 66604-1730
Here is a link to a map of their street address.
http://www.mapquest.com/maps/map.adp?country=US&addtohistory=&formtype=address&searchtype=address&cat=&address=3701%20Sw%2012th%20St&city=Topeka&state=KS&zipcode=66604%2d1730&searchtab=home
Here's their mailing address:
Westboro Baptist Church
P.O. Box 1886
Topeka, KS 66601