I have a Simply Rugged
Silver Dollar Pancake™- Leather Concealed Carry Holster made by Rob Leahy that is perfect for the Mdl 640.
The De Santis Thumb Break Scabbard
THUMB BREAK SCABBARD Holsters, Leather Holster, Ruger Holster, DeSantisHolster.com works great, too. Mine is marked 001-02, which I think is the size. From their web page, select
Smith and Wesson J Frame No Hammer 1 7/8" to 2 1/4" which is made for the Model 640.
I'm sure there are other great holsters out there, too.