Looking for a Colt upper receiver, stripped or with dust cover and forward assist.
Okay, I brain farted on your other thread(14" Colt barrel). One place to start is GandRTactical. They get a lot of Colt stuff through there. I was once on a wait list for a Colt upper. I got the notice but didn't respond(my gaze turned elsewhere). Put yourself on their 'Notify me'. They are very good about keeping up their notifications. I've gotten quite a few.
https://www.gandrtactical.com/Colt-M4-Upper-Receiver-Assembly_p_321.html