How can I find out the FMV of my property 10 years ago, the year my husband passed away. Related to if the $500K tax exclusion is applicable still if I sell my house this year.
This is a job for an appraiser. You can find a local (to the property) appraiser through Yelp or Google, or ask your favorite realtor for referrals. Appraisers are trained to valuate property and can provide valuation for a specific date.
This is a common practice, and your scenario (step up basis of an inherited property) is not at all unique. It would probably cost several hundreds of dollars though ($300-500, from my personal experience).
The IRS would want an authoritative estimate. Just relying on old sales records or advertisements will not be enough. Providing a formal appraisal by a licensed appraiser will satisfy them.