Start by checking the probate courts in the counties where they lived, died, or had property. Probate files are public record and you can get copies of the case files even if they didn't have wills.
If there are no probate files it's possible that they owned everything jointly with right of survivorship so when the first parent died the other parent inherited everything without going to probate court.
However, when the second parent died there would more likely have been a probate file even if he/she died without a will.
When you come back, please explain your situation and why you are asking. Do not use the preview button. That just seems to erase everything.