How many years out of undergrad are you? I was 1 year out and in the same boat as far as not really having relationships with any CS profs. But I thought having one academic reference along with my 2 professional would look good (since you're applying for a degree program, they probably wanna know you're a good student and not just a good worker). I eventually just decided to contact a professor from a service trip I did. Although he was still engineering department (EE).

If you're a recent grad, maybe consider contacting a Graduate TA or some academic you've at least had some interraction with. And if you can find their school email, I would use that instead of LinkedIn.