You need to be more specific and give details that indicate a particular family of Fosters. There are Fosters from all over the English speaking world. My particular Foster ancestors have descendants living in Texas and Oklahoma, but their particular branch moved from Cumberland County in Tennessee out to Oklahoma and Texas. Although I haven't been able to trace further back than Tennessee, my grandmother claimed that their family of Fosters come from Irish ancestry.
As the interactive map in this link shows, Foster lines living in America tend to be from quite a few states with some states being more common New World origin points than others.