I found two books that you may find useful. Depending on the look you want for your quilt and the colors you are going to use (modern or traditional) these may be of help.
http://www.amazon.com/Beautiful-Wild.../dp/1604600039 which has more realistic flowers and
http://www.amazon.com/Flower-Festiva.../dp/1571205292 which has 50 more stylized flowers. Remember, you can always enlarge any photos you find to the size you need.