Actually, to be able to specialize in your undergrad is actually kind of rare. Most programs will just have a degree in psychology, you can then take a bunch of social psychology.
This is the list I found off Princeton review
American Military University
American Public University
Bard College at Simon's Rock
Bethany College (WV)
Clarion University of PA
Eastern Nazarene College
Keene State College
Maryville University of Saint Louis
Mid-America Christian University
Northwest Missouri State University
San Francisco State University
State University of New York at Albany
Tennessee State University
If you'd like specifics, try collegeboard.com and click "Find Your Match." Across the top a list of criteria will show up, and from there you can type in "Social Psychology," click "go," and once you follow the directions, just hit "see results" if that's all your criteria for colleges.
most colleges off that. almost any college that has psychology programs will have social psychology as an option.