If the hammer and/or sear need to be replaced, are they still available? Or would I have to search for "new old stock" parts? And what kind of prices are they?
It's for a 657 no dash.
This 657 may have a target style wide hammer and trigger that are flash chromed. These can be found but are usually very expensive (.500 wide). If it has the narrow stainless hammer and standard trigger you are in for a tough search (.400 wide). There weren't a lot of this type made and used to start with and S&W no longer has them and while they can be found once in a blue moon, I would rate them as very rare. The standard triggers in stainless can be readily found as K, L, and N are all the same. You would be better off if both need replacement, to use color cased parts. I looked for a mint stainless narrow hammer for nearly 3 years and finally found one on Ebay this last December. Until I could find it I installed a Color Cased hammer and trigger in my 629-1 3". You may end up doing the same.
When I contacted Numrich, they showed the stainless 400 wide hammer or rather a hammer FOR the stainless but I ended up sending them back when they arrived as they were color cased parts not the flash chromed which is what they should have been. Color cased N frame parts are a relatively easy find. Getting them in near new or new condition is not always easy but it is possible. On the other hand people advertise the .500 wide target style hammers and triggers that are flash chromed on GB, Ebay and lots of other places including this forum. They are usually at unreasonably high prices. Start with an Ebay search for S&W stainless hammer and another for S&W stainless trigger. Keep in mind K, L, and N triggers are the same. You should be able to come up with a standard trigger for $25-35. Target versions can be up to double and sometimes triple that. On hammers the target versions can be $100 or more. Many of the 657, 629 no dash guns had the .500 wide triggers and hammers.
If you can find someone who has a good Bridgeport mill and knows what they are doing, it is possible to recut the hammer notch if it hasn't gone too far or been polished away. However, based on what you said, it sounds as though someone worked on the hammer notch and polished the trigger sear down to too fine an edge so that both got shortened to the point that they will not hold. First things to check before getting into the repair are the mainspring and the strain screw. Make sure you have a full strength S&W mainspring and not some after market or that the original has been thinned down by grinding or polishing or bent which is common in home grown jobs. Then make sure the strain screw is full length and not ground off so it doesn't put sufficient pressure on the mainspring. Those two parts can be had, new, from Brownells most of the time. Too light a pressure on the mainspring can result in the same problem you have described. However, it is more likely that some home gunsmith fibered the hammer and/or trigger trying to so a action job. But check the other items first. If you have a good High definition camera (not a phone) take a picture of the revolver with the side plate off in the cocked position and send it to me by email. Then if you are comfortable with doing it, take the hammer out and get me a good sharp focus picture of the hammer notches from the side and from the bottom and from the front. If the pictures are sharp, I can probably give you an idea of what your options might be and what parts you need.
What you probably should be looking for is a certified S&W revolver armorer. It's been a long time since LE Dept's carried revolvers so it may be a hard find. Call an descent size LE Dept and ask to speak to their armorer. Ask him if he knows of a revolver armorer. Those of us that were in that business usually know all the others in the surrounding areas because we help each other out all the time. There may not be one as there aren't any departments of any size in the area you are looking. You might have to go to Denver or go to Salt Lake City to find one.
To be honest with you, your best and cheapest way to go is to return the revolver for a full refund and then look for and buy one that is in good shape without the bubba action job, if that's what happened to it. 657 no dash guns are not that hard to find, although sometimes sellers think they are made of Platinum not stainless.