Either on ebay or possibly at apple store but not likely you need to get a mini VGA to VGA adapter. It should be one of the last inputs before the safety lock. Unless it is one of the newer iMacs the alunimum models it will be a Mini DVI to either VGA or DVI, but if you get the DVI you will need yet another adapter. But if its just one of the plastic white intel imacs it will be mini VGA.