The details are on the Movie Firearms Data Base
Stranger Things - Internet Movie Firearms Database - Guns in Movies, TV and Video Games
They say
Season 1: Smith & Wesson Model 66, stainless steel, .357 Magnum, 4" barrel, walnut grips (I just found this photo online, it's not the actual prop)
Here's a screen shot showing the reverse side of the revolver, notice the grip on that side has a concave cutaway called a "speed loader cutout"
Here's a pair of early 1980s walnut grips where you can see the difference between the two grips. The left grip has a "speed loader cutout"
(The airsoft pistol Allen Hawk linked to earlier is a dead ringer for this S&W66)
Season 2: Coly Python, nickel plated, .357 Magnum, 4" barrel, checkered walnut grips with gold Colt medallions (I just found this photo online, it's not the actual prop)
A screen-shot showing the reverse of the revolver. You can see that the grip on that side has the convex curve like the one on the other side, it doesn't have a "speed loader cutout"
I see that due to the popularity of Rick Grimes prop Colt Pythons in nickel are widely available, but they have the 6 inch barrel, not the 4 inch barrel of the Hopper Python.
Here's a resin one on Amazon; we would have to chop the barrel.
EDIT: What I ended up doing was buying one of those $25 Rick Grimes airsoft Colt Pythons with the 6 inch barrel, taking it completely apart, removing every ferrous bit so that it can pass through a metal detector, chopping the barrel, and gluing the whole thing back together.
Here's the process.