Find a farmer who will sell you some from his dryer. If you plant Roundup Ready soybeans that is what you grow. I have been planting them in food plots for years with great success. I pay $10 for 50lbs of soybeans to replant.
If you want clean seed you will have to find a seed rep who will sell left over seed. Just remember when you do this you will have to over seed as some of the old seeds wont germinate.