YANBU
In defense of people like the OP, if you've never worked in retail or know anyone that has, you won't be familiar with the system. It's only from this thread that I've learned that it's not possible to just get an item from out the back. The assistant really wasn't explaining why properly, her answers were vague which came across as unhelpful even if she didn't mean to be.
I think the assistant could've handled it better. She could've said it's not possible to get items from out the back as they need to go through the scanning system, but she could get someone to find out when they'd be available. It's not a big thing, but it makes a huge difference to how a customer perceives things. If an assistant had said to me, "well you don't have to shop here do you?" I'd be 

Firstly, why ask someone on the tills about stock in another department or going to get you something? That's clearly not the job they're doing right then
Depends on the shop. Many of the smaller places I've used only have a couple of people on the tills and no-one else on the shop floor, so there isn't any option that no ask one of them.
One step away from "Computer says no"
LOL, isn't it just. I wonder if she eye-rolled too
why should a young woman be expected to apologise on behalf of a large multinational company because their policies don't enable her to give good customer service?
Because when you're at work you're an ambassador for that company. She wasn't wrong that she couldn't get OP what she wanted, but she handled it badly and was downright rude at the end. People boycott companies all the time due to stuff like this. That's why employess have to watch their Ps and Qs regardless of their personal feelings.
I always tell customers to go elsewhere. Not in that manner. But I will say "have you tried next door?" Or "suchaplace may have it"
As a customer I've had that. My response was always an appreciative, "thanks! I'll try there". It's all about how something is phrased, tone of voice and body language isn't it.