I've had that twice in my life....once was at Chicos (women's clothing for those who aren't familiar). They have their own sizing scheme and I asked some information about how their sizes correlated to the "normal" size scheme I was used to - only to be rudely told that they didn't carry anything in my size. That was the last time I have ever walked into a Chicos and will never do so in my lifetime.
The second time was in Bloomingdales. I had just received a sizeable monetary gift and I was in a shopping mood. I went into Bloomingdales specifically looking for a winter coat, again only to be told rudely that they don't carry anything in their store in my size.
Both times, I was wearing US size 18 women's....I wasn't asking for the top end or low end of the general range.
Overall, I've come to a realization that if you don't fit a certain size demographic, many stores just don't want you anywhere in their general vicinity, like you might taint the store so badly that no "normal" person would ever shop there. I am on a weight loss journey and will be needing to shop frequently along the way....I've made note of the stores that don't want my business and will continue to shop accordingly in the future.
I've gotten some bad treatment because I apparently didn't fit the store's demographic. One time it happened I was in a Claire's looking at jewelry. I went up to the counter and asked to see a ring, and the girl working there said, "Oh, these are for young girls. Are you looking for a gift?" I said no, it was for me, and she asked, "You sure you still want to see one?" The manager, a guy in his 20s, shooed her away and showed me the rings, and I bought two. As I was leaving he was chatting with the clerk in a low voice in the corner of the store as someone else manned the register. I hope he was telling her that their demographic was customers, period. When I was there I wasn't the only gray-haired woman in the store who wasn't with a teenager.
Another time was the reverse: I saw some very nice scarves on some mannequins in the window of a petite shop. I have never been petite in my life. Anyway, I went inside, and the clerk asked, "May I help you?" I told her about liking the scarves, and she led me to a rack covered with scarves, and showed me a rose and pewter one and told me it was her favorite, and I bought one. I also bought some socks. The socks and the scarves were probably the only things in the entire store that would have fit me, but the clerk was totally professional.