I feel your pain. I suppose all you can do is try your best. If you genuinely know another shop stocks something, or the directions to another shop, opening hours etc. there is no reason not to help. If you can access google in your role you could do that for them.
But yes, I remember as a 16 year old doing my first job on a checkout, someone bought lychees. I'd never even heard of them, let alone seen one, and struggled to put it through the till. The customer was horrible to me over it. Lets belittle a working class girl trying to do A levels and earn minimum wage over lychees.
When I was a supervisor, a young woman who worked on the tills had a customer ask a question about homogenised milk before going on a rant about how thick people were at Tesco and no one would even know what homogenisation is let alone answer his question. I took incredible joy in pointing out that the woman who had held her tongue to you is actually in a summer break from her Chemistry degree at Oxford (one of the biggies, it was 25 years ago) so probably knows more about homogonisation than you and the rest of the staff put together, and that perhaps he should consider paying her and the rest of our staff a little more respect, and we will be happy to help find you the answers you want.
Another time I used the store's outdoor cashpoint on Easter Sunday at 11am-ish. There was a queue of people waiting to get in despite the store being closed. I decided to be nice and told them that I worked there and that the shop was closed until tomorrow which would be BH hours. Cue a flood of questions and comments. "My sister said it was open" Well sorry your sister thought that, but I promise we're closed. "Are you calling my sister a liar?" No, just misinformed. "I only need bread and milk" Try the express 5 minutes up the road "I shouldn't have to". Sorry, I can't open a Tesco extra on my own when I just came to get cash out. Another man joins the conversation "But I need to return this toaster!" You'll be able to do it tomorrow, but we're closed today. "I am not taking this all the way home again". At that point I'd had enough. People were literally following me to the car, and I quite firmly but calmly said "Look, I'm not working, the shop is closed. I just wanted £50 for my day out and didn't want you all waiting out here for nothing. I'm going now" Toaster man and bread and milk lady shouted that I was rude!
The point is, you can try your best for people, and its great when people have a bit of wider knowledge of products and other stockists etc. but you can't expect it all of the time. And what ever you do or say, some people will just be unreasonable arses.