Clint's character Blondie was using a Colt 1851 Navy .36 revolver that had been converted to fire metallic cartridges. Technically, such a gun didn't exist (from a manufacturer, anyway) at the time the movie is set. So much for historical accuracy.
About the closest thing available today as a standard production item is a repro of the Richards-Mason conversion of the 1851 Colt. Cimarron Firearms sells a nice one in .38 special. It carries an MSRP of $529, I think, so street price is probably somewhere around $400-450. It looks like this:
The main difference between the Richards-Mason and Blondie's gun are that the R-M has had the loading lever (rammer) removed and an ejector installed. Blondie's gun in the movie still has the loading lever and does not have an ejector. If you really want a recreation of Blondie's gun, probably the cheapest way to do it is to buy the Cimarron R-M and have a gunsmith remove the ejector and install a loading lever off an 1851 Colt cap and ball gun.