Replacement mid 2009 MBP battery

I'd like to replace the battery on my MBP. Iv'e changed it once already, its lasted for a year. But I'm struggling to find a genuine replacement can anyone suggest where I can buy one ? Also as I changed the original battery a year ago.. I can't find the model number for the original battery so I can chase up another one. I have a mid 2009 MBP 15". Model identifier Macbook Pro 5,3. 2.66GHz. Does anyone know the original battery info please ?

Thank you...Gerry