My first experience with IEMs was exactly the sort of demo, in a store, that you are looking for. But it was (at least) ten years ago, and I haven't seen anything similar since.
When Shure began marketing IEMs as stage monitors for musicians (I dunno, 8-10 years ago? Longer ago?) I was spending a lot of time hanging out at Sam Ash and Manny's here in Manhattan, because I had been DJing (really, really badly) at a bar near my apartment. One afternoon, I came across a Shure in-store display that was designed to demo the IEM monitors that they had just started selling.
They were hard-wired to a source that was contained within the display, and what you heard was a combo of sales pitch ("A revolution in onstage sound for the performing musician...") and music samples. Sitting on top of the display was a bowl full of foamies that you could attach to the cans, ensuring that you didn't end up wearing someone else's ear wax.
I didn't buy at the time, even though I was mightily impressed with the sound. They were EXPENISIVE! But the in-store display was perfect for getting at least a vague idea of what they sounded like. I haven't seen anything like this kind of demo since. Too bad.