Create Custom Product Attribute with Source Model in Magento 1.9.x

mysql4-install-1.0.0.php

<?php

$installer = $this;
$installer->startSetup();
$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$setup->addAttribute(Mage_Catalog_Model_Product::ENTITY, 'vendor', array(
    'group'             => 'General',
    'type'              => 'int',
    'backend'           => '',
    'frontend'          => '',
    'label'             => 'Vendor',
    'input'             => 'select',
    'class'             => '',
    'source'            => 'mymodule/product_attribute_source_vendor',
    'global'            => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
    'visible'           => true,
    'required'          => false,
    'user_defined'      => false,
    'searchable'        => false,
    'filterable'        => false,
    'comparable'        => false,
    'visible_on_front'  => false,
    'unique'            => false,
    'apply_to'          => 'simple,configurable,bundle,grouped',
    'is_configurable'   => false,
));

$installer->endSetup();
?>

Vendor.php

<?php


class Mynamespace_Mymodule_Model_Product_Attribute_Source_Vendor extends Mage_Eav_Model_Entity_Attribute_Source_Abstract {

    public function getAllOptions() {



               // $options[] = array('value' => 0, 'label' => 'Select Vendor');
        $collection = Mage::getModel('mymodule/vendor')->getCollection();
        ;

        $options[] = array('value'=>0,'label'=>'Select Vendor');
        foreach ($collection as $vendor){
            $options[] = array('value' => $vendor->getId(), 'label' => $vendor->getName());
    }

        return $options;
    }

}

?>