A cheap option is the Yongnuo range of flashes and triggers. I have a radio trigger that mounts to my hotshoe and I can mount a flash on top of that and have another flash off camera. It's been a while since I used them because I don't get to do a lot of portraits (and I am too lazy to get them out for other stuff). Not sure about compatibility with Sony but they have Nikon and Canon TTL compatible lenses (TTL uses the camera's light meter and adjusts the flash strength). Worth looking at.