Sunflowers look great whent they are green and lush - but are horribly thin once they die back. Corn is a better fall/winter screening option than sunflowers.
My sunflowers (in this picture) come from the Hardware Store - Bulk Birdseed Bin @ $.89/lb. We prepare the soil and hand broadcast the edges when planting corn in the Spring.
These pictures are of the same strip of sunflowers - different times of the year.
The corn is a much better screen - for a longer period of time. Sunflowers are good for 1-1-1/2 months tops in my area.
FH