Since it's easy to create natbib-compatible
bst files with
makebst, there are a lot of those out there. They can be recognized by examining the header. E.g., the following appears near the start:
%% The original source files were:
%% merlin.mbs (with options: `[long string of options]')
If your goal is to build an inventory (as opposed to matching a particular style), you can google for some of the strings you find there.
Edit: Apparently my answer wasn't satisfactory to everyone, because the answer has been reopened with a bounty. So let's be more specific: The following google query searches for bst files with the above header. It returns "About 203,000 results", which appear to be (or contain) real merlin-generated bst files. Even allowing for massive duplication, there's plenty to choose from.
"The original source files were" merlin.mbs with options:" +url:bst
And this is the same google query restricted to CTAN ("about 35 results"):
"The original source files were" merlin.mbs with options:" +url:bst site:ctan.org
If your goal is to match a particular style, you can of course just add the style or journal name to the above query.
If these results are still not what you're looking for, I think the question needs to be clarified: What is your purpose of looking for natbib-compatible bst styles? Why is this approach not satisfactory? There should be many other natbib-compatible styles out there (not created with makebst), especially ones derived from the explicitly supported ones. If unsure about a style, you can always just try it out.