To avoid a re-thread, here's one from the iRex' DR1000 forum:
Accessories discussion It includes the covers, but in the middle offers various comments on compatible pens ("penabled" technology) and where to get them.
Personally, I got iRex' Pro pen after comparing prices with some of the nicer Wacom pens. You can see a comparison between pens in
my Unboxing thread. It's the middle one. On the right is the Intuos pen for comparison, but you can't use those unfortunately.
Note that the extra button and eraser on top aren't supported yet, but may be in the future.