You may want to check out the Iron Range Research Center website a href="http://www.ironrangeresearchcenter.org/genealogy/collections/naturalization/">http://www.ironrangeresearchcenter.org/genealogy/collections/naturalization/> for naturalization records, including declarations of intent. Look under the Genealogy section and in the search box you can enter a surname. I found several of my families this way. If you have any questions, let me know.
Naturalization records, including declarations of intent, performed in local courts are at the Minnesota History Center. Those done in Federal courts are located either in Kansas City or in Chicago Regional Archives. Most are on microfilm, and can be borrowed thru your local public library, too.
But if you are looking for records before statehood in 1858, you may have to check the local court record books. Some of those have been published in the Minnesota Genealogical Journal.