For any build, you can buy your own case and send it in to PCS to have them do the build around it if they don't stock the case you want.
Pretty sure they only have the option for sending in your own case I'm afraid. You can always phone them to ask, but I suspect the answer would be that they can't offer that. What mobo did you have your eye on out of interest?
And what's your prospective build, budget, and intended use? I'm assuming you're looking at an AMD Ryzen system.