Yes, it should be port glass. I don't know who sells it here but I believe it's quit expensive, though it doesn't need to be that large. Make sure it's angled so it doesn't reflect any light back. Google turns up a few links:
I used this glass for my port, in 3mm thickness. Very inexpensive, flat, and AR coated. Do not angle the glass unless you want to reflect away a lot of light output (90 degree angle to the light gives the best light transmission).