The Esri partner network will let you search for Utility Network certified vendors and solutions, but to my knowledge there are only a handful on the marketplace and I believe one of the prerequisites to becoming certified is one or more reference sites in production. I am aware of a few staking vendors who aren't certified (yet?) who have implemented on the UN or who are in the process of implementing on the UN.
That search will return any partner who has the words utility or network in their name. What I recommend is a search for anyone who explicitly has a Utility Network Specialty certification from Esri: https://www.esri.com/en-us/about/partners/find-partner/search?specialty=Utility+Network+Management