Most likely they'll simply say you need a new keyboard - hardly anybody bothers to repair keyboards or mice these days, even laptop keyboards.
You might be able to talk to someone in the service department or another Apple repair centre who can sell / give you a space key off of a broken, non-working computer of the same model, but they are quite fiddly to put back on (one reason to dislike the new aluminuim keyboards 🙁).
Another option might be one of those companies that sell replacement keys for use in certain applications (e.g. Photoshop), but they are expensive, you'll end up with a pile of keys you don't need, and it's unlikely the arrow key will be part of any set.