There are some good solutions above, and mine is just a combination of two (I came here looking for a solution).
@Sarvar Nishonboyev's solution is good because it maintains the creation of the form input label and help-block for error messages.
I went with:
<?php
use yii\helpers\ArrayHelper;
use app\models\Product;
?>
<?=
$form->field($model, 'parent_id')
->dropDownList(
ArrayHelper::map(Product::find()->asArray()->all(), 'parent_id', 'name')
)
?>
Again, full credit to: @Sarvar Nishonboyev's and @ippi