If you remove the grips, you will see the type of grip frame on the casting web. All but the very earliest old model Blackhawks used the XR3-RED grip frame. The very early flat ops used the slightly smaller XR3. New model Blackhawks use the XRN3-RED frame
You'll find several great options for grips for either grip frame listed on eBay and Gunbroker, just to name a few. And if that's not enough, there are several custom grip makers (including myself) that make custom grips from exotic woods.