Locating a marriage record for an unknown place and unknown time is unlikely, unless you can come up with some more specific information. Suggest you do some systematic research to narrow down the time and place of the event:
1. Locate Grace and William in all the censuses from 1910-1940. This will give you an idea of where they were when, and where and when they could have gotten married.
2. Obtain Grace and William's obituaries. Obituaries usually give parents' names, place and date of marriage, and other useful info.
3. Obtain Grace and William's death certificates. These provide parents' names and more.
If you want a search of Wisconsin marriage records, contact the Wisconsin Vital Records Office: http://www.dhs.wisconsin.gov/vitalrecords/
Be prepared to pay.