Tressa,
Here in Ohio, Serpent Mound is the crown jewel of effigy mounds. You might try them. You can link to them from the Ohio Historical Society Web site, www.ohiohistory.org. Also, Mound City, dating to around the same era as Cahokia, would be a source for you. Their gift shop contains several books that might help you out. You can reach them through www.nps.gov. Hope that this helps!
Jim Sturgill