Cheapest solution is to use the built in Sony wireless system - uses the pop-up flash as a master to optically trigger remote Sony flash units - full TTL, etc
The downside is that the pop up flash may contribute to the overall lighting, and that the remote flash must be able to 'see' the pop up flash fire (this will work bounced off a white wall, etc, but gets less reliable as the route gets more convoluted!).
The next alternative is to use radio triggers - I use Pixel Kings, which are not cheap, but give full TTL & HSS, and have been happy with them.
You can go with cheaper triggers, but lose HSS (or are just manual only, for the cheapest ones).