What does it means "to get us by"?

It's an example of the phrasal verb "get by", meaning to "meet the minimum requirements" to survive or accomplish something.

In your sentence, it effectively means, "A lot of us like a small, sleek computer case to meet our need to enclose our computer without any extra features but such a case can be extremely hard to find".

Your sentence appears on a website that confirms this meaning: it also says that the case "supports the bare minimum of computer" and describes the cases as having "no flashy bells and whistles" (archived at Internet Archive), showing that the cases provide basic requirements, but nothing extra.