They sell 1/4inch think sheets at Home depot now in their window fixing station. I think it is a new thing they are doing now. But, I am not sure if you would use the same stuff for a tank.
You will need at least 3/8 inch for that. Mine is 6x3x2. 4 instead of 3 might be OK. It is about $200 where ever you go. Just look up glass in your phone book.
I live in Reston and used Reston glass. Look for mirrors or windows in your book. There must be a glass dealer somewhere near you. Other than the book look online at any number of online yellow pages. The Washington Post has a good local one.
!/2 inch is going to cost a fortune and be very heavy. I hope you plan to reinforce the plywood joints big time.