Metz and Sigma (and probably some others) make P-TTL flashes - but I would stick with the OEM ones. I've got an AF360 and am quite happy with it. I had an AF540 for a few hours but it had an issue with zooming. (It was a Craigslist buy and I was able to return it.) I have an older flash that has a swivel head if I need that - but usually if I want to shoot in portrait, I'll just have the onboard flash trigger the AF360 wirelessly.
The AF360 is a good bit cheaper so I would give that consideration over a third-party flash, even though you give up the swivel head and possibly some strength over the top third-party ones. You also may have issues with third-party flashes when Pentax introduces a new body - some flashes have needed firmware updates in the past; not the case with the OEM ones.