The overflow tank is not pressurized but it is sealed to the radiator so that when the fluid gets hot and expands, the extra coolant is sent to the overflow tank and when the engine cools down, it sucks the fluid back in to the radiator. Unless you change the radiator cap, the next time the engine gets hot, fluid will be forced out of the radiator and nothing but air will be sucked back in. This air may disrupt the water pump and cause some problems.
If the old tank has a part number printed on it, then a search of the part number at Rockauto.com should provide a replacement.