A great place to start for accessibility-ready WordPress themes is the WordPress Theme Directory on wordpress.org.
WordPress Theme Directory (WordPress.org)The WordPress Theme Directory provides a Feature Filter that allows users to search through its database of free themes for accessibility-ready themes.
If you choose to use freemium, premium or paid WordPress themes, make sure that your theme has built-in accessibility features.
And if you decide to build a custom website for your business or organization, consult with the web developer first to make sure that any accessibility concerns are discussed and reviewed before starting your project.
A good starting point if you are building your own theme is the Underscores starter theme, which follows web standards and has a number of accessibility features built-in.