I need your help.  I bought an Aimpoint CompML2 scope to put on a post-ban EA 16" flat-top.  Even though I brought the rifle to the store where I bought the scope, the guy gave me a mount without a spacer.  So when I went to put the scope on at home, there was a perfect view of the front site blocking most of the view in the scope.  I had him order another mount with a spacer.  I didn't yet know the whole business about co-witnessing as I was going to use the optics only (I didn't know there was an option to use the iron sights).  I have now read the posts regarding co-witnessing (http://www.ar15.com/forums/topic.html?b=3&f=18&t=159966 and http://www.ar15.com/forums/topic.html?b=3&f=18&t=159966).  Awesome explanations!

Now my question:  What do I need to buy as far as a rear sight?  Where is the best place to buy it (websites)?  Is there more than one option?  I haven't a clue about any of this.  
     

I would go for the ARMS #40 rear fold down rear sight. WWW.ARMSMOUNTS.COM web sight.
Jack

The ARMS #40 is probably the most popular back up iron sight.  It springs upwards when released, and the spring tension will keep it there if you bump it on something.  Other options include the GG&G MAD or A2 BUIS, Knight's Armament 300 or 600 meter versions, or a cut down detachable handle.

Keep reading here, this comes up about twice a week [;)]

As far as sources, look in the Equipment Exchange, again under "sights and optics" for sights offered by individuals or dealers who support the site.

Thanks for the info guys.  I ordered an ARMS #40 from Eagle Firearms today on the EE.  I ask enough questions here that I want to help support the sponsors.  The price was right as well.