Colt never sold just the sights of revolvers as far as I know.
The best option is to simply make a new sight from steel plate.
Colt brazed the front sights in a slot on the barrel, which requires red heat to melt the brazing compound, which is a type of silver solder that usually melts around 1100 degrees.
This will destroy the finish and may damage the aluminum frame, so this is not advisable.
One way to do this is to have the stub of the old sight machined out, down to the bottom of the original slot, and make a new sight that's a tight fit in the slot.
Then use a lower temp solder like Brownell's Force 44 to secure it.
Another option would be to file the stub off flush with the barrel and make a wide, longer based sight and attach with Force 44.
The larger the sight base, the more likely it is to stay put.
As weagle99 said, replacement of barrels in aluminum framed Colt's is not an option since the risk of cracking the frame is just too high.
When dealing with an aluminum frame Colt, options for things like this with barrels are severely limited.