Greetings Adrianne,
Thank you for your rather unique question. I will be the first one to admit that my forestry expertise certainly does not cross-over to homeopathics. Therefore, I cannot be of assistance regarding the tincture itself.
Seeing that eastern black walnut (Juglans nigra) is a very common tree species in Illinois, it will be very easy for you to find and collect green black walnuts in public parks, golf courses, cemeteries, etc. (local forest preserve districts are a great place). Start looking for prematurely dropped, green black walnuts in early August. Moreover, I would say 99% of naturally established and planted black walnut trees will be free of pesticides.