A sentence

Without. I assume this is cream to protect from the sun. Cream (whether sun cream or not) is a substance. We don't normally use "a" or "an" before the name of a substance (like cream, butter, ice, water, lotion, meat, soap, ...).

Actually, you have another choice: Can you go and buy a bottle of sun cream, please?

I think that in case this cream is supposed to be sold and bought in some well-known portions like tubes or bottles of the standard size, an “a” might be used, too, like in: “Give us two teas and a coffee, please” meaning two cups of tea and one cup of coffee.
What’s your opinion?

In the context you describe, yes, "a" might be used, too.

I usually think of those as something to eat! Don't ask me why.