How do I find my deceased father's oil & gas royalities?

My father passed away several years ago leaving me to assist my mother with their finances. She left her home and moved closer to me and her forwarding address expired after 120 days. I've recently discovered some lost oil & gas interest checks in Kansas & Texas that were returned to the state due to no forwarding addess. How do I manage to acquire these interests and is there a way to search for other oil & gas interests that may be missing?