I am an advocate of spring planting - especially for bareroot stock. Spring planting alleviates some of the common problems we have in central Illinois such as fall frost heaving and winter desiccation.
Please visit the Illinois Nurserymen’s Association website http://www.ina-online.org/index.htm to find retailers in your area, or else “Google” hybrid poplars. I am sorry, but I cannot recommend individual retailers/nurseries through this forum.