quick question on over flow tank?

hey guys while doing a quick fluid check on my '75 dart i noticed that the over flow tank has "do not fill" written on it...?huh? so in further inspection i noticed that there is a crack on the bottom of the tank. when i fill the radiator with coolant and start the car a small amount of coolant leaks out of there. so my questions are is there anyplace that makes a stock replacement one or some one that has a NOS one...or there is alot of "universal" ones out there and would it just be easier to grab one of those and which one would be the best for a high school kid budget? thanks