Just go to your local glass shop. I live on the north side of town, there's a place called DeSola Glass not far from my house. They had two shades available, "Bronze" and "Gray". I forget which is darker.
If you don't know where any glass shops are in your area, let your fingers do the walking in the yellow pages.
BTW if you get tempered glass, 1/8" thickness is sufficient. Your cost will be around $40, maybe a bit less.
Also, I recommend glass over plexi.