I hate the v notch and need to replace it with the square notch rear sight from S&W. Any idea where to buy and what size? It's like there are 4 different sizes I just want the stock size

Nobody?

You need to measure it. Crank the windage all the way to one side then use calipers to measure the height. It's likely either .146 or .160. If you have the top rail on, .160 is probably the better choice

In general though, it depends on the front sight height and your zero distance. There's a rear height to front height relationship that needs to be considered as it affects point of aim and point impact when it comes to elevation.

S&W Part 191490000 - .160 Blue blade with windage nut/screws kit
S&W Part 191480000 - .160 Blue blade with white outline with windage nut/screws kit
S&W Part  040730000 - .160 Blue blade only

Look at Harrison's rear replacement.