Firstly, this? - "There's a bunch of them out there, so I assume there's no copyright issues, right?"
In *this* particular case you *might* be correct (see below) but generally speaking here - erase that statement from your mind forever. Foreeeever! We're still in an age where many aren't taking copyrights very seriously, so just because you see people doing something, do not ever assume they have consulted with an IP attorney and were given the green light. Many times, they just don't bother researching, or are unaware that they should be doing that. For the safety and longevity of your business (and yourself, as fines and legal fees are no joke), always do what's right and research your material.
That said, disclaimer time - I'm not an attorney. If you happen to know a cute one, though, send him my way.
OKAY! Secondly, there are some free crown vector images that you can get in SVG and PNG format that have been released into the public domain. BUT:
Detailed example - if you're browsing some public domain or CC0 vectors and you find a cute balloon or plain red heart, and the artist says here you go have fun no attribution necessary, that should be fine because it's fairly simple, and should be fairly safe. But for something more complex, like a detailed vector trace of a vintage advertisement, you can clearly see the artist is giving up their rights to it in the license on their website, but is the source they created it from truly public domain? If not, does the artist have permission to create that derivative work and transfer a license of its use to others? Are any of the symbols or names on that poster trademarked? Are they logos? And remember, copyright is international, and so is Zazzle. Is that ad public domain here but still under copyright elsewhere? You really have to ask yourself ALL of these questions. See? Told you it was ugly.
That was just a little background info that you may find helpful when asking yourself if something is okay to use. never just assume what people are doing on a commercial website is the right thing. Copyright ignorance is a thing, and even people skilled in navigating that territory can mess up accidentally.
Now onto your specific question - As for the particular image you're looking for, I'm unsure as to whether or not that crown is legally usable. Yep! All that info for an "I don't know." But before you strangle me, lemme explain:
It might be okay, but it's not something I've personally looked into, so I don't know. I know the vector is available for free, but I don't know the legalities surrounding using that actual symbol in commercial work. That's something you're going to have to research. Just FYI, the associated font commonly used for that design, if I'm not mistaken, is copyrighted, but free for personal use ONLY. I have seen it for sale with a commercial license for about £10(GBP). That font also comes with the crown symbol. Of course now I'm sitting here wondering if the crown vector was somehow traced from that font, is that legal, did they get permission, etc. See how deep it runs? I honestly can't give you a simple yes/no answer to your question because it involves so many variables - including where the fonts and images people are using are even coming from.
My suggestion - if you want to do keep calm posters without the hassle, pick a different image - one you are absolutely sure is fine to use with a license in writing that gives you explicit commercial permission and has no icky murkiness (or just create it yourself) - and use a different free commercial use font that again comes with a license. Alternatively, contact the company with the commercial license font, explain your usage, confirm permission for the font and symbol, purchase it, and use that.
To sum up everything in the simplest terms - you need to be able to establish and prove that the artist has given you explicit commercial permission. AND, especially if you're putting it up for sale, you're gonna want to be able to establish and prove that the artist has the right to even do that to begin with. Sounds simple, but it ain't!
Hope this helps!
The first (and only, mind you) time I slapped an image onto that zero I thought "what in the world...who...what...seriously???", and immediately trashed it without even glancing at the options. LOL I'm a dope.