In funeral scenes in movies, everyone is always dressed in completely black outfits. All the men are wearing black blazers, pants, shoes, etc. All the women wear black jackets, dresses, veiled hats, etc.
Who are these people that keep completely black outfits in their wardrobe for these occasions? Or is there some sort of business that you can rent 'funeral clothes' from?
It's not all that unrealistic. In real life most people in the western hemisphere do try to wear all-black for a funeral.
I'm not saying there won't be someone in a grey suit or navy blue dress because that's all they have. But most people do try to find or borrow black clothing. Men may not find all the items in their closet, but many women would already have black outfits --- because it's slimming and they may wear it for regular clothing too.
Also, many funerals we see in many movies tend to be in a comfortably-off family, and movie characters with money can afford to go out and buy a black outfit even if they think they'll never be at another funeral -- though in a normal life they will. These characters seemed quite well-off for money and if anyone didn't have it in their closet they could buy it.
When my parents died I had no problem wearing all-black as I'd been into black clothes, first for Goth reasons, then because it was slimming.
When being an extra for funeral mourner scenes, I basically could wear business formal. My casting director actually forbade extra from wearing all black. Actually, when doing background work, it is frowned upon to wear everything the same color.