There are some entries in the third-party flash databases. You need to check which ones support P-TTL.
Flashes (Third-party) - Pentax Camera Accessory Review Database Flashes (Third-party - Legacy) - Pentax Camera Accessory Review Database
Some of the gear databases (like for Pentax-branded lenses) are really thorough, but the third-party flash and accessories databases are a little thin. Take the ratings with a grain of salt.
You can of course go to B&H's website and filter for flashes that support Pentax P-TTL, and check review comments there.
Similar Threads 
