I’m always on the look out for really close to exact copies of stuff that’s “authentic”, and I saw this so let me know guys!
http://www.heatsinks.shopeio.com/inventory/details.asp?id=715&cat=Heatsinks&sub=
How close are you hoping to get? It does have a similar style/appearance, but it also looks a little big, there are nearly two times too many fins, and the big gap on both sides doesn’t look right for a gun heatsink(the spaces between the fins are all suppossed to be equal. A scratch-built heatsink would be easy enough to build, anyway. Just take a piece of wood or plastic of the right thickness and add glue on some fins (thin styrene would be one good way to do that).
However, if you’re only going for close or similar looking, it might work (though you’d probably have to adjust a few things on the gun body to accomodate it).
I didn’t see the measurements, and I figured one could Dremel off the side parts, I just noticed the fins, but I didn’t count them, but I just thought it looked like the original somewhat because of the style of it, most heat sinks these days are “cool” looking with different patterns and such. Just trying to see what I can find 
Yeah, you could remove the side parts, but then you’d also want to trim a little off the top, or else the width & height proportions would be off, and it would appear too tall and skinny.
You are right, though; it is a pretty close match compared to the appearance of nearly all other heatsinks out there. I’m just saying that with all the trouble one would have to go through to make it more accurate, it’d be easier & more efficient to just scratch-build one.