To change polarity you need a storage element (store energy and then release it in a some way) then capacitors or inductors, the first one is a switched capacitor solution (there is also another nice ic that can bve used MAX828 **broken link removed**), about the inductors solution you need to build a BUCK-BOOST converter, into the nonisolated topology you can easily achieve negative voltage from positive voltage source. Also I don't know what is your application and current requirement then I can't evaluate more about the MOSFET sizing and related rdson.