Renters insurance is no longer available through the university (neither through ASM (Associated Students of Madison) nor any other office). In the past, WSA (Wisconsin Student Association, former UW-Madison Student Government) offered a renters insurance policy to UW students.
UW Credit Union does offer renters insurance. More information can be found here: https://www.uwcu.org/products/Insurance/Default.aspx
To find companies offering this coverage, consult "Insurance" in the Yellow Pages.