For more expensive lenses I would only recommend these two.
However, for cheaper lenses or non-glass accessories, it pays to check out Amazon.com.
Coincidentally, I have the same issues when I got my K-7 a year ago! I was hunting around in Singapore for the Pentax 16-50mm and after calling the usual suspects for a few weeks ("Hello, do you have the Pentax 16-..." "No!") I succumbed to MSC's suggestion to go for the Tamron 17-50mm instead. Although I could find no fault with the 17-50, deep down I still miss owning the 16-50mm. Now that the price of 16-50mm has gone up, I have given up on owning one (I hate paying more for the same item).
As for switching brands, my advise is - determine what is your priority in a camera and follow it, and not follow the brand. The worst thing that can happen to you is to get stuck with a brand and watch all the other cameras from other manufacturers overtake yours while you struggle to get the picture you want which can be done more easily.
When someone asks me "What camera (brand) do you have?". I will stare at them blankly and ask: "For what purpose?"
Switching brands because of non-availability of lenses will short-change you in terms of utilizing your camera to its maximum potential.
One disadvantage which was not mentioned was that for Canon and Nikon cameras, since the base is larger, you have more reviews and hands-on information available in the web. Try finding a review for any Pentax lens and you will probably find only a handful of reviews. That makes choosing a lens for Pentax a much more challenging exercise than say the other mainstream brands.
I left K-7 for the 1D4 as the latter gave me what I wanted in terms of ISO sensitivity. But now that the K-5 appears to achieve the same performance with good DR and lighter weight (did I mention I like K-7 for the travel-friendly size and weight?), I am looking this way again.
Choose wisely, my friend.