Geolocation banner rotation

We need some code to plug in to a website that will display specific banner ads dependant on the geolocation of the user. ie - certain banners are only shown to US vistors, other traffic is shown an alternative... The banners to be displayed may be of varying size and be served from ad and cpa networks. This also need to be able to work within Joomla - not necessarily a full extension but able to function as some custom code within that framework. Thanks - look forward to hearing from you