Descendants of Ole Ulver in US

My great great grandfather Ole Benjaminsen Ulver (b. 1856 in Hitra, Norway) emigrated in 1907 to US, to work in South Dakota. Two of his sons, Ingvald Benjamin Olsen Ulver and Ole Konrad Olsen Ulver followed him in 1908. Ingvald got married to Ida Peterson (b. 1889 in Norway) and stayed in US. Ole Konrad went back to Norway abt. 1909. According to family members their father was determined to go back together with his son Ole Konrad. For some reason (he got ill?) he didn´t, and he never came back to Norway. There were some contact between the family in Norway and Ida and her grandson about 1967-1969. According to Ida there were no contact at all between Ingvald and his father after 1908/-09, and Ingvald believed that his father had gone back to Norway. Ingvald died abt. 1917. In Minnesota Death index I find that one Ole Ulver died in 1930 in Kandiyohi. My question: 1) How can I find out more about my great great grandfathers life in US? 2) How can I get in touch with any descendant of Ingvald and Ida (I have some surnames of their children and a couple of their grandchildren, in the sixties they lived in California, South Dakota and Florida). I will be grateful of any help or tips of how I can proceed my search. Line