Depending on how new it is, the best is to send it in to Colt.
Colt will work on almost all Colt firearms made since about 1950.
http://www.coltsmfg.com/custom-q10047-FIREARM_REPAIR.aspx
If it's older and Colt won't do it, talk to Cylinder & Slide Shop.
They're a world class custom shop that are quite good on Colt's. Prices are high and they're usually slow due to work load but quality of work is top notch.
http://www.cylinder-slide.com/
Another I've recently been told is good is Heffron's.
Since I haven't personally seen their work I can't vouch for them, but I've heard good things.:
http://heffronfirearmclassics.com/default.aspx
Fair warning: There's just about no one left who really understands the Colt's.
Many local gunsmiths will tell you they're experts and have the special tools needed for Colt's..... but don't.
You stand a very good chance of getting a Colt back in worse condition then when you left it because the gunsmith attempted to teach himself about Colt's on your gun and failed.
The recommend repair source is the Colt factory if they'll work on it.
Their prices are competitive with other top gunsmiths, and turnaround is faster than most.
