I am sure that there are other companies in your area, but one national company that does have locations in Georgia is Binswanger. Look around on the web for glass companies, many that strongly advertise auto/windshield glass repair work also have suppliers for plate glass and will gladly give you a quote for the piece you need. It is typically sold at a price per square inch. Unfortunately, I hate to say, once you get up to 1/2" thickness, panels are not cheap. I don't know what you call expensive, but it can't hurt to call around and see what quotes you can get! Just out of curiosity, what do you have invested in the tank as is?
Good luck.
Good luck.