I've had 8 or 9 shocks for K bikes pass through my garage. I got them on the bikes, and I've bought them new and used off of eBay and from on line sellers.
For a new shock, I have been very satisfied with YSS. They ask a few questions and set up a shock with the spring. The one I have on my 16 valve K100 is pretty nice and performs very close to the Fox Twin Clicker.
Fox Twin Clickers are nice, but no longer available new. Once you get it dialed in the ride is very good. There are sources on the interweb for rebuild parts, and they are not terribly hard to rebuild if you have a few tools like a spring compressor. They can be found on eBay for between $150 and $200.
I've had two Works Performance shocks. They are also good units and are easily rebuildable. Parts are also available for them on the web. I got one on eBay for a bit over $100. I had another that I had to rebuild. It cost about $90 to have new seals installed. Again, these shocks come up on eBay from time to time for under $200, sometimes a bunch less.
OEM shocks are okay, they keep the rear end of the bike off of the wheel. I have a good one here that I will use to replace the aftermarket shock on a bike when I sell it if I have to cut the price a lot.