When they are available, they go for about $13-15. You might check with Wegener Media. Their electronic part costs are high, but sometimes the trim part prices are not too bad. This part is I believe interchangeable with all the 5300c/5300cs/5300ce cases.

This port door was a bad design - they break again, even when you are careful with them. I have a 5300 that was never out of my office and one day the right side just went when I gently opened it. So...you might just remove the taped part and live with the exposed ports...