I recently acquired a 4" Colt Diamondback and am looking for a concealed carry type holster for it. It seems that nobody stocks these anymore. Even my go-to custom leather guy doesn't have a form/dummy to make one. I've placed ads in the WTB sections on several forums with no luck. I really can't afford the top end custom makers, nor do I want to wait months for one to be made.
Anybody know of a source?
I've heard that a holster for the Ruger SP101 will fit. Can anybody confirm or deny this?

If I remember correctly, the Diamondback was built on Colt's D frame which is the same as the Detective Special. The barrel contour is a little different as well as usually longer. Maybe your leather guy has a form for a Detective Special. An SP101 holster should be pretty close. For an OWB holster, a generic "Small Frame" holster from Uncle Mikes should work. The Don Hume JIT belt slide holster lists the SP101 and Detective Special as using the same holster so an IWB holster for a 101 should work. Diamondbacks are very nice revolvers. Congratulations on owning one.

You can afford a top-end revolver but don't want to kick for a top-end CCW holster? No offense meant, but that is strange to me.

It won't be for EDC, just occasionally when the mood strikes.
Besides, its a little quirk of mine that if a handgun is suitable for concealed carry, I like to have the gear available so that I can exercise that option if I want to.

Have you inquired about a Bianchi IWB? I'm referring to the old IWB made of suede with the belt clip. Pretty basic holster. I carried a Combat Masterpiece for a number of years in one with satisfaction. No too expensive (at least 40 years ago). Because of the barrel on the Diamondback the K frame size might fit it ok. Although the D frame Colt and K frame Smith are a little different size-wise it may be satisfactory for you to look for one of those. IF they are still being made..... Hope you find something for that gun. What a delightful revolver they were/are. Wish I had both of mine back... (2 1/2" and a 4").