You can use the value on the County Assessors Property Valuation you receive annually.
Also check your Homeowner's Insurance policy for an insured value.
You could ask a realtor to prepare a written statement for you, based on comparative market sales.
Here's some info to help you:
https://turbotax.intuit.com/tax-tips/rental-property/real-estate-tax-and-rental-property/L3e09vT71