Magento 2: store selector for custom module in admin form

magento\vendor\magento\module-cms\view\adminhtml\ui_component\cms_block_form.xml

<field name="storeviews">
    <argument name="data" xsi:type="array">
        <item name="options" xsi:type="object">Magento\Cms\Ui\Component\Listing\Column\Cms\Options</item>
        <item name="config" xsi:type="array">
            <item name="dataType" xsi:type="string">int</item>
            <item name="label" xsi:type="string" translate="true">Store View</item>
            <item name="formElement" xsi:type="string">multiselect</item>
            <item name="source" xsi:type="string">block</item>
            <item name="dataScope" xsi:type="string">store_id</item>
            <item name="default" xsi:type="string">0</item>
            <item name="validation" xsi:type="array">
                <item name="required-entry" xsi:type="boolean">true</item>
            </item>
        </item>
    </argument>
</field>

magento\vendor\magento\module-cms\Ui\Component\Listing\Column\Cms\Options.php

Copy this file in ur module & apply name changes accordingly

For your phtml file define your block class containing following code: Please refer getStores function in following block class

namespace Vendor\Module\Block;

class Classname extends \Magento\Framework\View\Element\Template {

    protected $_objectManager;


    public function __construct(
    \Magento\Framework\View\Element\Template\Context $context,
    array $data = []
    ) {
        parent::__construct($context, $data);

    }


    /**
     * Retrieve stores structure
     *
     * @param bool $isAll       
     */
    public function getStores($isAll = false)
    {
        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $model = $objectManager->create('\Magento\Store\Model\System\Store');

        $out = $model->getStoresStructure($isAll);
         $result = array();
        foreach($out as $data){
           $result[]=$data;
        }

        $_mainWebsites = array();
        $i=0;
        $j=1;
        foreach ($result as $k => $v) {
            $_mainWebsites[$i]['main_website']= $result[$k]['label'] ;
            $_mainWebsites[$i]['store'] = $result[$k]['children'][$j]['label'] ;
            foreach ($result[$k]['children'][$j]['children'] as $p => $q){
                 $_mainWebsites[$i]['store_views'][$p] = $result[$k]['children'][$j]['children'][$p]['label'] ;
            }
            $i++;
        }

        return $_mainWebsites;
    }

}