Well, they may all be slightly different, but I had the same problem with one of mine, went to Home Depot, Lowe's, Locke Plumbing Supply, ABC Plumbing Supply, and two other hardware stores. My faucets have no brand name, model number, etc. that I could find. I finally found "similar" but not the same parts at a hardware store. But a year or so later, I had the problem again. And that time, I happened to notice the damage to a rubber o-ring. I discovered that all I ever actually needed was a new o-ring for a few cents from Ace Hardware. Some of those top caps are metal and are screwed on, some are plastic, and just snap on, but they have the same problems and the same fix.
Now of course that's the experience I've had; your problem may be different.