These are even rarer than the 100-300/4 now, in the second-hand market.
I got mine when Jessops sold their last remaining stock, maybe a little over a year ago, for £429 if I remember correctly . And to think I accidentaly stumbled upon them when I mis-clicked on an old search results page which was still in my "favorites" tab: there they were, 3 of them, right on top of the page (had they been listed at the bottom I may never had seen them).
I think when they come up on Pentaxforums they're usually priced around $1000, but since the last time I saw one I think second-hand prices for the Voigt 125/2.5 and the A*/FA* 200/4 have gone through the stratosphere.
Your best hope now is if Sigma finally release their 150/2.8 OS in Pentax mount, which is supposed to be even better than the 180/3.5

Il like this type, I have the same - more expensive - branded Sun Sniper, but it's pretty much the Black Rapid design (OK, it's fleabay but it's really cheap):
http://www.ebay.co.uk/itm/Camrock-Quick-Strap-QS-1-Pentax-K-7-K-5-K-r-K-x-K-m-K7...

PM sent.

I believe you can buy a generic sync connection cover for a couple of quid off eBay, if I'm not mistaken - those things are a nightmare, I don't know where mine went.

PM sent.

Hi Roy,

I think I'll take it, someone here in France was just looking for one, I'll bring it back for him from London next time I come to visit.
I'm sending you a PM.

PM sent.

Still available, open to offers.

Is this the OS version?

Bear in mind that, from the feedback I've received from people using a converter with the DA*300, the autofocus becomes so slow that it is nearly useless for birds in flight, for instance (I know some people who sold the converter back, even moved to Nikon or Canon to get a real "wildlife" setup). However, they were very satisfied with the image quality, so it's rather good for slow-moving or static subjects.