That's a pretty small supply in a pretty common output.. There should be fully off-the-shelf solutions for isolated supplies in that range. If you need custom, the next step would be to find a controller IC intended for that application. There is a good chance you can find an app note for a complete solution, perhaps including transformer part numbers if you take a look at some manufacturer's websites. TI would be a good one, intersil, linear and several others would be good too.
Even if you need to go totally custom you don't want to re-invent the wheel, app notes and datasheets are a fantastic source of information for something like this.