Years ago, you used to see articles in the gun magazines on "How to install your own target hammer and trigger in S&W revolvers".
The articles made it SOUND like these were "drop-in" parts, and you could just stick them in and be good to go.
The trouble is, the hammer and trigger "often" DO install with little trouble, BUT NOT ALL THE TIME.
In most cases at least some fitting is required.
Shortly after these articles appeared, I'd get a wave of S&W revolvers with non-functioning actions that had brand new hammers and triggers.
Another factor: S&W has made changes to the internal parts over the years, and parts OFTEN are totally wrong and won't work.
While you can buy new hammers and triggers from parts suppliers like Gun Parts Corporation, Jack First, Brownell's and others, you simply can't guarantee that the parts will fit or work safely.
As above, send it in to S&W for a 100% RIGHT job.
Plus S&W has the correct parts and will install them at prices as good as most local gunsmith's.
Bottom line is, this is one of those things just enough people get away with to temp everyone else.
You're better off getting it done right the first time.