I bought the cheaper eBay battery holders for less than $10 each from China. I have 2 of them for less than half the price of the official Pentax one which work exactly the same. I find that my K-50 works fine on either power source but my K-r was very temperamental. Just make sure you use decent batteries like Energizer Lithium's (pricey but worth having a set or 2 as a backup). I can get over 800 shots to a set and they hold their charge for years so keep a set in your bag and you'll never run out of power.
I also use Sanyo Eneloops normally and get about 350 shots to a set. Since I switched to rechargeable batteries I also bought a decent intelligent battery charger which helps keep batteries in a better condition.
Good luck and keep us posted.
---------- Post added 16-06-14 at 09:26 ----------
Originally posted by ScooterMaxi Jim Personally, I'm not a fan of the adapter - even with new Eneloops or the like. You still have a voltage gap, and under high strain (continuous shooting), the AAs tend to drop down - then recover after a minute or two. My advice is to go with at least two spares of high quality or OEM, and reserve the adapter for lithium batteries as emergency back up. The battery situation is the biggest downside with this camera if you are shooting a major project.
ScooterMaxi Jim, I haven't experienced the drop under continuous shooting but I don't run to the buffer often when I use my camera. Most of my photography consists of short bursts. Good point though. I think I was so put off the stock battery after my problems with the K-r that I stopped trusting the performance I used to get.