Like Pamjb said,
Alamo and National are the "official" agency on-site, but there are numerious agencies at the DTD hotels (Dollar @ Grosvenor,
Avis @ Hilton, Hertz @ Shades of Green are the ones that come to mind). I do not believe that Alamo or National have an office in Ocala, more likely Avis and Hertz will though...