My husband and I asked his parents and my parents for addresses.
My parents reached out to some family members for me to collect addresses to reduce the many things I needed to do for planning. His parents had everyone’s address in their address book. We just went over for dinner and copied the addresses one night.
We also had some addresses from sending Christmas cards or visiting the family. Depending on your celebrations and traditions, sending some cards - Thanksgiving - Christmas or another important tradition you can test out the addresses to make sure they get to them safely. When it’s time to send your invitations you will know if you need to check any addresses with certain family members. Or you can send Save the Dates in advance to test out addresses.