Hi Andrew,
Depending on the county where you are or where you want to buy notes, their records can be searched online or you have to go to the Recorder of Deeds office. Good place to start https://publicrecords.netronline.com. Click on the state and corresponding county link you want to find Office of recorder of deeds or something similar. Some counties may require subscription. (not all counties are online). Unless you want to target a specific county and be one of the first to reach them, buying list from a list provider may be better use of your time. Reach out if you want a detailed walk through.
Thanks
Sri
571-482-7257