Does anyone know where to get a cover glass of an aquarium of 33 gallon flat back?

it is very difficult

33 tanks gallon (assuming you have the common size, which is about 48×12 inches) has a hood for a 55 gallon aquarium. You do not appear in the list of 33 gallons. The only difference between 33 and 55 gallons is the height.

My new 20 gallon long aquarium update # 1