transformer selection part is always baffling, digikey cant provide with the spec i want or lack thereof. this is to step up 5Vdc input to ±12V to ±18V (bipolar Vdc) output. power rating is around 6-12W preferably small size such as 1x1x1cm unit, or probably more if science doesnt allow such small size. but i saw small unit such as this..

but usually they are getting power from mains 110-220Vac input. i need 5V input. the topology is something like in attached picture (transformer in the black lined box). 3 pins input centre tapped. centre pin will be Vdc+ input and both transformer end inputs will be alternatingly switched to create Vac effect. and then on the output, 2 pins only producing 12-18Vac, and later to be bridge rectified to get ±Vdc output. 3 pins centre tapped output also acceptable. toroidal type suggestion is also welcomed, and higher power transformer of this kind too are welcomed. any hint on transformer selection method, supplier or model number? thanks.