I bought
this generic L Bracket by Benro for my K-5 a long time ago, and it also perfectly fits the K-3 (it's more snug on the K-3, actually). It's the same L-Bracket you see here in one of the images from the Pentax Forums
Sirui T-025x In-Depth Review:
Just one thing to note - if you heavily use the side panel (USB/HDMI connections), you will find them inaccessible once you mount the L-Bracket as it doesn't line up properly (as a dedicated mount would).
If I were buying a new one right now, I'd probably get
this one as it seems to be a little better, including an anti-rotation flange. Mind you, slipping/rotating on the above L-Bracket is VERY rare, so you can't really lose either way. And they are both really cheap compared to what L-Brackets usually go for (at least the dedicated ones). I can't tell you if it gives access to the side panel, however my assumption would be no as well. Because I never use it, I've found that this minor niggle has no impact to me.
Regardless of which you get (if you choose to follow my advice), I'd recommend also replacing the mounting screw as the one included does not have a D-Ring that would allow you to twist it with your fingers - you need a coin/screwdriver:
Hope this helps
-Heie