There is one listed on ebay right now; bidding ends in 3 days. Motherboards have a short manufacturing life, so finding some new is impossible after 6 months. Ebay is your best source.
The GA-8I945PM-RH is an OEM motherboard. That means it was built specifically for a major OEM manufacturer, such as HP or Gateway, etc. This means there is no manual for it except for whatever the OEM provides, and Gigabyte doesn't support it directly like a retail motherboard.