\Bitrix\Main\Loader::includeModule('sale');
$orderId = 1;
$dbRes = \Bitrix\Sale\Internals\OrderTable::getList(array(
'filter' => array(
'ORDER_ID' => $orderId
),
'select' => array('SOURCE_NAME' => 'TRADING_PLATFORM.NAME')
));
if($tpOrder = $dbRes->fetch())
$sourceName = $tpOrder['SOURCE_NAME'];