I have been looking and looking for a 6/3 rear sight for my removable carring handle, but can not seem to find any.  Does anyone know of a good source?  Thanks

You need to contact Bushmaster.  They have the parts you need and will sell them to you.  They don't list them on their website or in their catalogs.  You need to contact tech support or customer service for the part numbers and call in/email the order in.

Here's the info they sent me via email, but I would confirm this info before placing your order:

Complete A3 Carry Handle assembly    # 9349063-9---------------$99.95

Carry Handle, Forged     # 9349063-2------------------------------$42.95
   Clamping Bar      # 9349063-3------------------------------------$3.95
   Cross Bolt, 2 required     # 9349063-4----------------------------$1.95 each
   Thumb Nut, 2 required     # 9349063-7---------------------------$2.95 each
Rear Sight Base      # 9349074-1-----------------------------------$21.95
   rear sight flat spring     # 8448536---------------------------------$0.50
   rear sight aperture     # 9349075-----------------------------------$7.95
   rear sight windage screw    # 9349076-----------------------------$3.95
   rear sight helical spring, 2 required   # 9349069--------------------$0.50 each
   rear sight ball bearing, 3 required   # MS19060-4808--------------$0.50 each
   rear sight windage knob    # 9349077------------------------------$5.95
   rear sight windage knob pin, retaining   # MS 16562-98------------$0.50
   index, elevation     # 9349066-1------------------------------------$4.95
   knob, elevation     # 9349067-1------------------------------------$4.95
   index screw      # 9349065-----------------------------------------$0.95
   index spring      # 9349063-8---------------------------------------$0.95
   elevation spring     # 9349070---------------------------------------$1.95
   pin, spring retaining     # MS 16562-121----------------------------$0.50

If you bought this as a stripped carry handle off of ebay (there have been tons of them being sold there lately), you will need from Rear sight base on down.  The only parts that are 6/3 (carry handle) specific are the sight base, elevation knob, elevation index, index spring and maybe the elevation spring.  All the rest are standard, milspec A2 sight parts you can get anywhere.  Chances are you will pay more for the sight assy than you did for the stripped carry handle (assuming you got it on ebay), but you probably spent less than the $99.95 that they want for the new handle.