Night sights are available from a number of sources, including XSSights, PT (IWI), Meprolight, Trijicon, and the factory. (The factory sights may be Meprolights or PTs.)
Some of us have had bad experiences with PTs in the past, but they may have gotten their act together -- a number of ownership and management changes have taken place. Their warranty is good.
You can install them yourself (with proper tools, which is a big deal), have them installed locally by a gunsmith, or send the slide to the sight maker (depending on the maker). Sending the slide is cheap -- as its not considered a gun -- and requires only regular mail and insurance.