You're about to spend a wad of money and earn yourself a load of trouble.
First, in all likelihood a different cylinder will not cure your problem.
As above, they're made to a specific specification so you won't get a tighter throat.
Second, any cylinder you find will be a used cylinder, so it may have other problems not immediately apparent.
Third, revolver cylinders DO NOT "drop in".
To fit a different cylinder to a frame, you MUST have a cylinder assembly consisting of a cylinder WITH a fitted ejector.
You can't just stick the old ejector in the replacement cylinder since it won't be fitted to THAT cylinder.
A used assembly will have already been fitted to a different frame, so it will not properly fit your frame.
Just because a cylinder will fit into the frame and close does NOT mean it actually fits the frame.
Among other things a replacement cylinder and ejector assembly must be fitted and checked for:
Barrel/chamber alignment on ALL six chambers.
Timing on all six chambers.
This is no job for anyone but an experienced S&W qualified pistolsmith.
Pick up a cylinder somewhere and somehow get it to fit into the gun and you'll have worse accuracy then you do now, if not an unsafe gun.
In your case, all I can suggest is talking to one of the hard chrome plater services that offer plating of the chambers.
Possibly they can plate the chamber mouths to decrease the size.
Armoloy of Fort Worth offers chamber and bore plating.