Add this to your template file:

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function YOUR_THEME_NAME_preprocess_views_exposed_form(&$vars, $hook) { //this is important

    // only alter the required form based on id
    if ($vars['form']['#id'] == 'views-exposed-form-where-to-buy-page-1') {

        // Change the text on the submit button
        $vars['form']['submit']['#value'] = t('Search');

        // Rebuild the rendered version (submit button, rest remains unchanged)
        unset($vars['form']['submit']['#printed']);
        $vars['button'] = drupal_render($vars['form']['submit']);
    }
}