If it has to be Colt, here's a good deal...free shipping too!
Colt AR15/M16/M4 20rd USGI Alum Mags
https://www.armsunlimited.com/Colt-AR15-M16-M4-20-round-USGI-Aluminum-Magazine-p/sp62103.htm
Colt AR15/M16/M4 20rd USGI Alum Mags
https://www.armsunlimited.com/Colt-AR15-M16-M4-20-round-USGI-Aluminum-Magazine-p/sp62103.htm