This is the one I use:
http://www.cabelas.com/product/Willi...h-All+Products
But the one Sabot uses is a better one for adjustments
Whichever you get, I would definately get the "twilight peep appeture" that Sabot mentions. The peep that comes with them is OK for daylight shooting at the range, but you need a larger appeture for twighlight in the field.
Both of those would be mounted in the rear most holes (scope mount holes).
There is also a Tru-glo Globe sight comes with a "ghost ring" that mounts in the forward open sight holes. Out to 100 yards, I don't think there is a lick of difference in the ghost ring and the peep, but for longer, the rearward peep should shine.