Does anyone know of any bereavement groups for an elderly spouse and daughter in Mankato, MN?

Call any hospice organization they would let you participate in their bereavement groups. Many do not require you used that hospice organization.

If loved ones were under hospice care at the end, hospice offers grief support for free for one year.
Also the support group Grief Share is all over the US. Often those groups meet at local churches and they are free as well. You can "Google" to see if you have some in the area needed.

Many churches have grief groups that they run or host. They don't need to be a member of that church or faith to participate.

Contact their local Area's Agency on Aging for resources.

Join Nextdoor.com and ask their local community of neighbors for recommendations of groups or counselors.

You may want to do a search on the Grief Share site:
https://www.griefshare.org/

Most funeral homes sponsor bereavement groups in my area.

They might do well to contact the funeral director who took care of the arrangements.

I’m not familiar with your area but I wonder if you contacted a hospice if they could help you find a group. Churches sometimes have them. Also you might check with your area agency on aging. Also if you look on psychologytoday.com you can search by area and issues for therapist. If you search for ones covered by Medicare their office might have groups to recommend.
You are kind to help them.