American’s Walking Club
They have an extensive database of walks and hikes across the Unitied States. When searching for a walk there is the ability to select only walks that are wheelchair accessible.

To find a wheelchair accessible walk, first go to this page https://ava.org/find-by-state/ Then pick the state you’re interested in. The next screen will allow you to start filtering the walks. In this example I picked Washington State. In the bottom left of this image you can see I checked the Wheelchairs option. This will limit the walks listed to those that can accomodate wheelchairs.