How to get products by stock qty?

You need to create a join with the cataloginventory/stock_item table to get the stock qty for a product, you could use:

$collection = Mage::getModel('catalog/product')
     ->getCollection()
     ->addAttributeToSelect('*')
     ->joinField('qty',
         'cataloginventory/stock_item',
         'qty',
         'product_id=entity_id',
         '{{table}}.stock_id=1',
         'left'
     )->addAttributeToFilter('qty', array('gt' => 0));

ONE WAY:

$stockIds = Mage::getModel('cataloginventory/stock_item')
    ->getCollection()
    ->addQtyFilter('=', 30) //can be ->addQtyFilter('>=', 30), depending on requirement
    ->getAllIds();

$products = Mage::getModel('catalog/product')
    ->getCollection()
    ->addIdFilter($stockIds)
    ->setPageSize(10);

SECOND WAY:

$oCollection = Mage::getModel('catalog/product')
->getCollection()
->joinField(
    'qty',
    'cataloginventory/stock_item',
    'qty',
    'product_id=entity_id',
    '{{table}}.stock_id=1',
    'left'
)
->addAttributeToFilter('qty', array('eq' => 30));

The 'FIRST WAY' in @TBI Infotech's answer wont work, as the ->getAllIds() method returns the stock id, not the product ID. Instead you need to add this;

$stockIds = Mage::getModel('cataloginventory/stock_item')
->getCollection()
->addQtyFilter('>=', 30); 
//->getAllIds();

foreach($stockIds as $stock) {
   $idarray[] = $stock->getProductId();
}

$products = Mage::getModel('catalog/product')
->getCollection()
->addIdFilter($idarray)
->setPageSize(10);