The USDA statistics are good for proso millet, and they are updated regularly.
USDA National Agricultural Statistics Service. 2020a. Crop Values 2019 Summary (February 2020) https://downloads.usda.library.cornell.edu/usda-esmis/files/k35694332/9w032m10t/hq37w596k/cpvl0220.pdf
USDA National Agricultural Statistics Service. 2020b. Crop Production 2019 Summary (January 2020) https://downloads.usda.library.cornell.edu/usda-esmis/files/k3569432s/sj139j59z/1257b842j/cropan20.pdf