Kirsten, when I made a black and white quilt for my son I spent over a year collecting fabrics. It is much harder to find black on white than the other way around. The only advice I can give you is to keep looking and collecting. Here are a few websites for you that have some with white backgrounds. Hope it helps you.
http://www.fabricparadise.com/cgi-bi...8&View=All&v=1
http://www.thecalicocat.com/fabricsgraphicdesigns3.htm
http://www.trinityquilts.com/categor...ategory_id=137
http://www.quiltedfox.com/fabricbw.htm