The flash extender mentioned in the instruction manual (part number A1787305A) may be available for purchase through Sony® Parts and Accessories Sales. Go to Parts and Accessories.
NOTE: The flash extender is not necessary when using the SEL16F28 or SEL1855 lenses that are bundled with the NEX cameras for sale in the United States. These lenses are small enough so they do not cause unwanted shadows due to the light from the flash.
A better alternative to using a flash extender, however, is to use the HVL-F20S external flash. The HVL-F20S not only extends the flash range, but also offers bounce flash for shadowless indirect lighting. It may be available for purchase from the Sony Store.
Please contact a local or online authorized retailer to purchase a Sony product.
For customers in the United States: https://www.sony.com/retailers